I understand that there are several reasons politicians consider when they decide about sending military or financial aid to Ukraine. But in the end, these aids are financed by US/UK tax payers. The national debt of the US is already quite high, especially because of the big spending in the last years to mitigate the effects of COVID. Are there mainstream politicians or political organisations in the US or in the UK who argue that this kind of spending must be reduced due to possible financial problems in the future, even if this may lead to some let's say partial win of Russia in the war?
With respect to the United States,a proxy for politicians against supporting Ukraine is the vote record in the US House of Representatives for the S.3522 - Ukraine Democracy Defense Lend-Lease Act of 2022. Only 10 members of the House of Representatives voted against the bill. All of which were members of the Republican Party. It's not viable to go through the histories of each, but here is a quote from Matt Gaetz who was one of the ten that voted against the bill:
As only 10 members of the GOP voted against the bill, there are no major political organizations against the support measures. Whether the individuals from the votes against the support measures are mainstream or not is highly subjective.
I think the closest an MP in the UK has come to this sort of criticism was Jeremy Corbyn, former leader of the Labour Party who now sits as an independent in Parliament. In a speech to the People's Assembly Against Austerity on February 12th - note that this was before Russia invaded Ukraine - he made the following remarks:
I say to this, to the Treasury Building over there, to the MPs over there; I've always noticed whenever they want money for arms, for weapons and war, it's always there. When you ask for money for benefits, when you ask for money for housing, when you ask for money for councils and the health service - "you've gotta be very careful, we can't afford this".
A nation that can afford to spend so much on nuclear weapons and other weapons, and be prepared, wrongly in my view, to be involved in a war in the Ukraine, can well afford to support the poorest communities in this country.
Senator Rand Paul, while he seemingly did not vote against US trade sanctions on Russia (incl. oil), which passed 100-0 in the Senate (in April), was apparently opposed to just gifting weapons to Ukraine, as quoted in March by Fox News:
Sen. Rand Paul, R-Ky., weighed in on the White House's request to Congress for $10 billion in aid to Ukraine, saying he's not opposed to selling weapons to Ukraine, but the U.S. should be careful about giving weapons away and expanding on national debt, which would threaten national security. [...]
Paul responded to the White House's ask, saying he's "not opposed" to selling weapons to assist Ukraine, but he's "opposed" to giving away weapons and expanding upon the United States' trillions of dollars in debt, which would "threaten our own national security."
"I’m not opposed to selling defensive weapons to Ukraine. I am opposed to giving away weapons or money to buy weapons as I believe that we should not expand our $30 trillion debt that threatens our own national security," Paul told Fox News Digital.
I'm not sure how he voted on specific bills relating to that kind of military aid though.
It turns out it would be a lot harder to parse the positions from votes before April, because the bills for supporting Ukraine with aid were tacked/combined with other appropriations that elicited opposition.
On the other hand, Rand Paul and 10 other Republican Senators voted against the $40 billion aid package for Ukraine, in May. CNN reported in one of those live feeds that make it hard to link to individual posts:
All Democrats supported the legislation. Eleven Republican senators voted against it: Marsha Blackburn of Tennessee, John Boozman of Arkansas, Mike Braun of Indiana, Mike Crapo of Idaho, Bill Hagerty of Tennessee, Josh Hawley of Missouri, Mike Lee of Utah, Cynthia Lummis of Wyoming, Roger Marshall of Kansas, Rand Paul of Kentucky and Tommy Tuberville of Alabama.
However, from another CNN report, it seems that Rand Paul opposed this bill mainly for lack of a spending accountability mechanism to his satifaction:
But Sen. Rand Paul, a Republican from Kentucky, blocked passage of the aid package until Thursday, which is the day the Biden administration said additional funds must be approved to avoid a lapse in support for Ukraine.
Paul has demanded that language be added to the bill that provides a special inspector general new authority to oversee how the Ukraine aid is being spent. And while members from both parties broadly agree with that notion, forcing a change to the bill at such a late stage is time consuming and would slow getting the needed emergency aid to country.
Under Senate rules, any one senator can slow down the process. It took about a week to overcome Paul's objection through timely procedural steps that the majority leader had to take on the Senate floor.
"I think we should have an inspector general," Paul told CNN earlier this week. "We have one out there and overseeing Afghan waste. He's been very good at it. You don't have to wait for an appointment. He's got a team up and running. And I think that's what we should do."
So it seem he's changed his reasoning for opposing compared to earlier reports... somewhat. He also repeated however (last week):
Paul said in a floor speech before objecting to the legislation's passage last week that his "oath of office is the US constitution not to any foreign nation" and, "we cannot save Ukraine by dooming the US economy."
I also managed to find his taped remarks on that occasion and he summed up the military aid to Ukraine to $60 bn, which he then said it that it exceeded the yearly military budget of any country outside the top 5 spenders. And he's made various other comparisons with cancer research spending, the State Department budget, etc. So, yeah, he thinks the US spends too much on this war.