I think these quotes from a Belgian and German NGOs kinda summarize the situation of peace activists in these circumstances:
“We had maybe three, 4,000 people, which is not many,” Ludo De Brabander, a member of Belgian peace group Vrede vzw, told The Intercept. “It was difficult to mobilize.”
“Iraq was very clear: It was an aggressive war based on false arguments,” he added. In Ukraine, by contrast, it was Russia that had staged an illegal, unprovoked invasion, and U.S.-led support to Ukraine was understood by many as crucial to stave off even worse atrocities than those the Russian military had already committed. That has left peace activists scrambling, said De Brabander, “because we don’t want to support NATO. And of course, we also oppose what Russia is doing. And a position in between, with alternatives to war, is very difficult to sell.” [...]
De Brabander noted that it did not help that some on the radical left of the peace movement “see only U.S. responsibilities or EU responsibilities.” That has exposed more moderate voices to the accusation that they are apologists for Putin. “There’s this very black-and-white vision that if you’re not with us, then you’re against us,” he added, noting that those calling for the dissolution of NATO were regularly accused of defending Russian interests. [...]
“We have been calling for the delegitimization of NATO, and there is really no reason to change that,” said Reiner Braun, a German activist and executive director of the International Peace Bureau. Braun noted that a coalition of dozens of groups calling for NATO’s dissolution are planning a peace summit in Madrid in June, to counter the alliance’s official gathering in the same city. “The main reasons why we are against NATO, the militarization, the military spending, the aggressive attitude, NATO’s expansion — these are all criticisms that are still valid.”
“We are definitely in opposition to Russia’s invasion of Ukraine, but without excusing Putin, we are also explaining that one of the reasons for the current situation is NATO’s expansion over the last 25 or so years,” he added. “It is not an excuse for the invasion, but it helps to understand how such a situation could happen.” [...]
It’s what De Brandeber describes as a “policy of self-fulfilling prophecy”: NATO taking a provocative action (expanding to Russia’s border) that contributes to a crisis that, in turn, justifies the existence of NATO. “Putin has become the best defender of NATO policy,” he added. “He made NATO very strong with this war.”
I've read that (pretty long) article in its entirety, but it alas doesn't contain any quotes on a concrete proposal how to de-escalate now, unless you want to read between the lines that NATO dissolving should also mean Western countries not sending weapons to Ukraine. Save maybe for this quote:
Antonio Mazzeo, an Italian journalist and peace activist [said] “But it’s true that a majority of political voices and pundits have become uniform.” He added, “There is a whole segment of the population that rejects the logic of war, of taking sides, of sending weapons, but it hasn’t figured out how to take a position, how to directly intervene in the discourse around this war.”
Condemning the "illegal war in Ukraine" but also condemning NATO was the position of one representatives (Judith Benda) of the (German political party) Die Linke to that parallel summit in Madrid. Likewise, Sevim Dağdelen, an MP for Die Linke said in an interview
alongside my party Die Linke, I have spoken out against arms deliveries from the very start, because we think they will prolong the war and loss of life in Ukraine. [...]
Although this may seem utopian to many people in Germany today at this moment, our long-term goal has to be to dissolve NATO and replace it with a collective security system that has disarmament and cooperation as its overarching objective.
So, if you want a reflection of those NGO position (quoted earlier) in a political party, Die Linke is probably a good example.
And while the idealistic position in re NATO possibly sets them apart (e.g.) from the AfD, in that interview Dağdelen also mentions pragmatic reasons like Russian gas:
The economic war with Russia is jeopardizing Germany's entire prosperity model. The Western sanctions have not ended the war. Instead, they are acting like a boomerang. They are hitting us, the people and industry in Germany. [...]
Should gas deliveries from Russia cease, we will face a disaster in Germany such as we have not known since the global economic crisis during the Weimar Republic.
Which have also been raised by some AfD members, like Höcke. Somewhat aside, interestingly, the AfD seems split on how strongly to condemn the war, and these developments somewhat contradict what Eike Pierstorff wrote further above--the June AfD conference failed to muster a majority to condemn the war.
The "Rethinking Europe" resolution called for improved ties with Moscow and only referred to Russia's invasion of Ukraine as a conflict, never using the German word for war, Krieg.
It was put forward by the party's influential and arguably most extreme right-wing figure, Björn Höcke, but encountered resistance from more moderate forces in the party and its leadership.
Delegates from western Germany said the resolution's text trivialized the invasion and should use a stronger term like Russia's war of aggression.
Despite the party's two co-leaders, Tino Chrupalla and Alice Weidel, requesting that the text be revised, they failed to achieve a majority by two votes among party delegates.
So the conference was called off early. That article also mentions that AfD has also opposed sending weapons, and that that has apparently cost them votes in the western side of Germany:
[The AfD] has also suffered regional election losses in western states this year that many in the party attribute at least in part to its comparatively ambivalent posture on the war in Ukraine, which include its opposition to Germany sending weapons to the government in Kyiv.
Opposing heavy weapon shipments apparently was something that the AfD could more easily agree upon:
The motion [to send heavy weapons] would prolong the fighting in Ukraine and "could make us party to a nuclear war," said the AfD's most senior Bundestag lawmaker Tino Chrupalla.
The socialist Left Party [Die Linke] was also against the move, pointing to earlier statements by Chancellor Olaf Scholz about how heavy weapons deliveries increase a risk of a nuclear escalation. [...] The Left Party also said that delivering more weapons would simply make the war last longer.
Now if you want popular sentiment, according to an April poll
The poll – a monthly survey of political sentiment for ARD and die Welt – also shows that supporters of the Greens, SPD, FDP and CDU/CSU are overwhelmingly in favour of the arms deliveries, while the majority of AfD voters are against them.
(Die Linke is alas not mentioned. Nor does the [English] coverage of that poll extend to reasons why those who were opposed were so.)
You probably know the situation in France better than I do, but it seems that one can generalize to a good extent from the one in Germany to other European countries, e.g. AP headlines "France’s Le Pen warns against sending weapons to Ukraine" (but sadly there are no direct quotes in the article). Likewise, according to AP "Mélenchon has also expressed opposition to supplying weapons to Ukraine and wants France to leave NATO’s military command, ideas which are not acceptable to center-left politicians." (But not direct quotes, nor detailing his reasoning for opposing weapon deliveries.) In Italy, the 5 Star movement is apparently also opposed to sending weapons to Ukraine. And "Italian transport unions on Friday [May 20] striked in protest against military aid for Ukraine and called for an immediate ceasefire". In Spain, Podemos (which is somewhat like a Spanish version of Die Linke) is part of the coalition government, but they oppose NATO membership and sending weapons; they actually put out a statement criticizing the deliveries, but [insofar] didn't quit the government over that. Vox [typically classified as populist right] however seems to have endorsed arms shipments in February, but I could not find any newer statements.
I suppose we can add Donald Trump here now. Although his statement is a little vague on specifics, he says he'll force a peace deal between Russia and Ukraine within 24 hours of being re-elected president. I'm not totally sure if this is a different speech [the wording is a bit different] or another section of the same, but he was also quoted as saying:
“It really has to be done from the office of the president,” Trump said. “And you have to get them both in a room and there are things you can say to each one of them, which I won’t reveal now, which will guarantee that this war will end immediately.”
What Trump meant by
"I could have negotiated. At worst, I could've made a deal to take over something, there are certain areas that are Russian-speaking areas, frankly, but you could've worked a deal."
is anybody's guess.
Now for something more explicit, some Republicans led by Matt Gaetz have proposed a resolution (which has a pretty long preamble but ultimately says) that:
(1) the United States must end its military and
financial aid to Ukraine; and
(2) the House of Representatives urges all combatants to reach a peace agreement.
Among the reasons listed/claimed is the vast amount of money that the US spent on that aid (spelling out all of that takes most of the preamble), depletion of US armament stocks "weakening United States readiness in the event of conflict", and that
by providing assistance to Ukraine, the United
States is inadvertently contributing to civilian casualties.
As for their record on other similar matters (i.e. how pacifist they are in general), Reason paints a pretty mixed picture. Some of [but not all of] these Republicans have voted to also stop military aid to the Saudis, but apparently none did that when it came to Israel.
Generally speaking, even critics of "America First" doctrine[s] seem to admit/conclude that it includes a pacifism component.
As I see nobody mentioned Corbyn here... (I managed to find a quote from him without paying 6 pounds to the Russell Foundation, which published an essay from him earlier in the war, next to one from Putin, by the way.)
“Pouring arms in isn’t going to bring about a solution, it’s only going to prolong and exaggerate this war,” Corbyn said. “We might be in for years and years of a war in Ukraine.”
Corbyn gave the interview on Al Mayadeen, a Beirut-based TV channel that has carried pro-Russia reporting since Vladimir Putin’s invasion of Ukraine.
“What I find disappointing is that hardly any of the world’s leaders use the word peace; they always use the language of more war, and more bellicose war.”
He added: “This war is disastrous for the people of Ukraine, for the people of Russia, and for the safety and security of the whole world, and therefore there has to be much more effort put into peace.”
According to Silvio Berlusconi [who seems far more overtly sympathetic to the Russian position that others I mentioned above tough], the the US could pressure Ukraine into an immediate (next day) ceasefire by promising Ukraine a 6-9T$ "Marshall plan" for reconstruction, while at the same time threatening to cut all aid in the alternative. He also says the war is Ukraine (actually Zelensky's) fault for "attacking the Donbas republics". When asked whether it would take two parties (i.e. Russia's agreement) for a ceasefire, Berlusconi answers "no", so presumably he's talking about some kind of unilateral ceasefire by Ukraine, although admittedly he's isn't too clear on that angle.
And Prigozhin maybe, assuming RFERL isn't making this up (as I've not seen it reported elsewhere insofar):
In a statement published on Telegram on April 15, Prigozhin urged Moscow to declare its goals in Ukraine as "achieved" and bring an end to the fighting there. Prigozhin wrote that "many of those who yesterday supported the special operation today either have doubts or are categorically opposed to what is happening."
Actually it has been reported by Newsweek too, with more detail
He also wrote that "theoretically, Russia has already achieved this decisive end by eradicating a large part of Ukraine's active male population and intimidating another part of it that has fled to Europe."
"Now there is only one thing left: to firmly gain a foothold, to claw in those territories that already exist. But there is a slyness - if earlier Ukraine was a part of the former Russia, now it is an absolutely national-oriented state," Prigozhin added. "If before February 24, 2022, the European Union was greedy to give Ukraine tens of millions of dollars, now tens of billions are being turned off for the war."
So he seems to be acknowledging a stalemate on some level, but he's calling it more of a victory for Russia.