In theory, all 50 Democratic Senators can use their vote to demand something from Chuck Schumer or other Congressional leaders. But in practice we only hear about Joe Manchin following this strategy and he seems to be getting away with it quite well. But why don't more Senators follow the same tactic for the sake of their state? Is Joe Manchin the only Senator who's got very strong electoral prospects and doesn't need to worry about consequences at the next election?
He isn't the only one-- Sen. Sinema has certainly made her fair share of demands. But swing votes generally have much more power because they could credibly threaten to defect.
Sen. Manchin is a Democrat that represents a very, very Republican state (West Virginia is R+22 in the Cook Partisan Voter Index). As such, he can very credibly suggest that he won't vote for a particular measure unless it is changed to suit his preferences. His voters aren't going to punish him at the ballot box for holding up some legislative accomplishment, they're going to be more likely to vote for him because he opposed something they see as overreaching.
Occasionally, you'll see demands coming from the far left wing of the Democratic party (or the far right wing of the Republican party) who can credibly threaten to vote against a bill that is seen as too centrist. Sen. Sanders and the Congressional Progressive Caucus have very occasionally threatened to sink a bill unless it was changed to their liking. But given that members of these wings depend heavily on votes from moderate voters within their own party, they would face much greater chances of a backlash at the voting booth if they killed (or threatened to kill) a bill the mainstream members of their party supported.
If you're a moderate Senator within your own party, you have extremely little room in which to realistically threaten to vote against a bill in order to gain a concession. You depend heavily on moderate voters and donors from your own party. Moderate voters of your own party (presumably) support the bill in question. And, presumably, the moderate Senator themself actually supports the bill in question. Threatening to vote against a bill you personally like that your constituents like and that the donor base likes is no way to win re-election.
That being said, a 50/50 split does make it much easier for everyone to make relatively low-profile demands. It's a lot more likely that a moderate Senator is going to get a small change inserted into a large bill during the drafting process that is beneficial to an industry in their state when the Senate is divided 50/50 and every vote matters. That moderate Senator likely has plenty of ways to derail the bill without coming out and threatening to vote against it if he or she doesn't get her way.
First, your implication that he does this for personal benefit is suspect, though it is a favorite talking point among certain groups (who construe things as benefitting industries he and his children are heavily invested in in order to benefit themselves, rather than as a consequence of other factors). Traditionally Manchin has long been a spanner in the Democratic works in part because he has consistently refused to budge on a bill in exchange for benefits his constituents would benefit from.
Second, Manchin is a Senator for the state of West Virgina. This is a heavily red state that voted for Trump over Biden by more than a 2-to-1 margin. That a Democrat holds a Senate seat in this state is almost mind blowing. The reason is that he has a long history in the state, having been a very popular Governor before becoming a Senator, and he is well-known for being one of most conservative Democrats around. He knows his state and his constituents well, and he pursues legislation and policies that suit his voters well.
Third, Manchin is most likely serving more as a spokesman of centrist Democrats. There's a lot more than just two (Manchin and Sinema) in the caucus, but most of them have a much stronger Democratic and Progressive base of constituents. This means they have to be more careful about upsetting those voters overly much, and are incentivized to keep their objections to more progressive agendas behind closed doors and at the negotiating table, rather than out in public view. Manchin and Sinema provide them a convenient privacy curtain, by being the face of centrist demands and concerns. Manchin, if anything, benefits from what you might otherwise think is negative PR among Democrats; and Sinema...well, frankly she just doesn't seem to care, despite having been voted in on a progressive wellspring, and we'll see how that pays off for her in her next election.
So it's basically all a win-win situation for the Democrats, all other things held equal. They cannot possibly hope to hold Manchin's Senate seat with any other candidate, and they cannot possibly hope to hold the Senate currently without him. He is necessary to control the Senate. And he gets to be the Democrats' internal boogeyman, to the apparent delight of his voters (which increases his odds of getting re-elected as a Democrat, again to the party's benefit), while masking a lot of the internal divisions within the Democratic caucus that might otherwise make for even more embarrassing PR issues if they were played out in the open.
It's important to remember that Manchin is not necessarily the only politician to oppose some of his party's plans, he's just the most visible. This is a common tactic, where the most outspoken critic of an idea from their own party becomes the de-facto leader of the "opposition" (which can include non-vocal opponents). Even amongst the leadership, there may be a larger number of folks hoping some party push efforts go away without having to vote on them
Joining Manchin in early opposition to court-packing and ending the filibuster was Sen. Kyrsten Sinema (D-Ariz.). But now that legislation to add four justices to the Supreme Court has officially been introduced, even more Democrats are coming out against the idea.
In fact, shortly after the court-packing bill was introduced in the U.S. House in mid-April, Speaker Nancy Pelosi (D-CA) rejected the far-Left’s push to swiftly vote on the court-packing legislation, stating: “I have no intention to bring it to the floor.”
There's a real danger for Democrats in overplaying their hand as well. Manchin holds more than just votes here. In 2001, a longtime Republican switched parties in a 50-50 Senate
Two days later Jeffords, who died Monday at age 80, would leave the Republican Party that he served in on Capitol Hill for the previous 26 years, first in the House and then the Senate, and would caucus with Democrats. The move ended an historic five-month run in which the Senate sat deadlocked at 50-50 margin, with Vice President Dick Cheney giving Republicans titular control as the tie-breaking vote. Jeffords was handing Thomas A. Daschle (D-S.D.) the title of majority leader and putting the breaks on the domestic agenda of the Bush White House.
Manchin could switch parties (his state is deeply Republican and would likely welcome such a move), which would change who the Senate Majority Leader is. That would end any such talk of ending the filibuster or using more Reconciliation bills.
They do get special stuff just for their state, they just don't have to be as dramatic about it because usually what they're asking for is fairly neutral to the rest of the party so they can just get it with minor negotiation--sure we will fund your library/historic site/cultural thing/road improvements, if you agree to fund my (equally unobjectionable) project in return. But what Manchin is asking for is pretty objectionable to most of the other Democrat senators (and the people who vote for them), so he's having to make much larger waves to get what he wants.
The name for this is pork barrel project, and there are several groups which keep track of this so you can easily catch up. For example, Citizens Against Government Waste group has a "Pig Book", and some highlights from 2022 are:
The 30th installment of CAGW’s exposé of pork-barrel spending includes: $240,000,000 for the M1 Abrams upgrade program, opposed by the Pentagon; $31,500,000 for two earmarks funding presidential libraries; $10,171,000 for 24 earmarks for the Save America’s Treasures grants, including $500,000 for preservation of the Nansen Ski Jump Historic Site in Milan, New Hampshire by Senate appropriator Jeanne Shaheen (D-N.H.); $3,000,000 for the Palo Alto Museum by Rep. Anna Eshoo (D-Calif.); $1,000,000 for the St. Louis Symphony Orchestra by Senate appropriator Roy Blunt (R-Mo.); $650,000 for feral swine management by Senate appropriator John Boozman (R-Ark.); and $240,000 for rodeo drive improvements in the town of Chico Valley by Sen. Kyrsten Sinema (D-Ariz.).
(The full list is very, very long, not to mention this happens in local government, too, where, e.g., a state legislator may get a special provision in a bill that would benefit their local area, or even a city legislator may find some project that happens to benefit their neighborhood.)
This is speculative, but here are some aspects that probably weigh in on this strategy:
He is expected to be skeptical of, and even oppose, "progressive" Democratic initiatives, by his WV constituents: 68% Trump in 2020 elections. Contrast that with a senator from say California - their constituents would be much more likely tar and feather them for opposing a locally popular bill, making their bluff more likely to be called by Biden.
Conversely, giving some pork to take home gives him political cover. Manchin needs to retain his seat (which is pretty useful for Democrats in an absolute mathematical sense given a 50/50 Senate, despite repeated claims to the contrary here and in the press). A major concession, pro-fossil fuels which is especially important in that particular state and given this particular bill, with local jobs, like the pipeline, helps keeping him electable. Biden may have calculated that playing absolute hardball, even were it possible, would endanger that Senate seat in the future. Look at poor Liz Cheney, she did the right thing, but her tribe cast her out. Manchin has a lot of local strengths, but his last election was not won by a large margin:
Manchin won the November 6 general election, defeating Morrisey 49.57%-46.26%
The same logic applies to other Democratic congress members in more conservative-leaning states.
p.s. I couldn't resist adding this blurb about the wiki article about the primaries, just to illustrate what registered WV Democrat voters' priorities are:
In 2018, Manchin ran for reelection. He was challenged in the Democratic primary by Paula Jean Swearengin. Swearengin is an activist and coal miner's daughter who was supported by former members of Bernie Sanders's 2016 presidential campaign. She criticized Manchin for voting with Republicans and supporting Trump's policies. Manchin won the primary with 70% of the vote.
When a politician threatens to withhold their vote, the threat is to prevent a particular bill from passing. In other words, if the bluff is called, the outcome is the status quo. Nothing changes. Such a threat is far more credible when the status quo is acceptable to the politician.
To the extent that centrism can be defined, it could be described as the position that the status quo is broadly acceptable. This inherently gives centrists more leverage in this bargaining position, because by definition, they don't really care if the bill doesn't pass.
By contrast, someone at the wings of a party typically wants change, and typically members of the same political party want change in roughly the same direction. Thus by blocking their own party's legislation, they are blocking change in the direction they agree with. This is a much weaker bargaining position, because if their bluff is called, the status quo is a worse outcome than a change that is in the right direction but insufficient.
In other words, it's a game of chicken where one participant is in a tank and the other is on a bicycle. It's pretty easy to see who will flinch first.
Joe Manchin is a Senator from West Virginia... West Virginia is one of the hardest leaning 'red' states in the US... Joe Manchin is an anomaly being a Democrat from that State.
The only reason Joe Manchin has resisted the Democrats up to this point is to appease his electorate, that is overwhelmingly Republican... The Democrats greased the wheel for Manchin by providing specific benefits to the State of West Virginia through the energy sector...
Make no mistake... Joe Manchin has been playing politics the whole time.