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(CNN) —  

With just 15 words, Joe Manchin ended the possibility that Joe Biden’s first term would live up to the hopes that liberals had for it on everything from gun control to voting rights to even, possibly, the size and scope of the President’s $2 trillion infrastructure bill.

“There is no circumstance in which I will vote to eliminate or weaken the filibuster,” wrote the West Virginia Democratic senator in a Washington Post op-ed published Wednesday night.

Which is pretty dang airtight when it comes to pledges. And which means, assuming Manchin sticks to it, that Democrats do not have the 50 votes they would need to change the Senate rules to eliminate the legislative filibuster. Which, in turn, means that the idea of passing meaningful gun control legislation or moving forward on the election reform bill passed by the House are simply not possible.

But, it’s more than that too. While the big headline of the Manchin op-ed is his opposition to ending the filibuster, he also makes clear that he disapproves of using “reconciliation” – a budget tool that allows bills to be passed with a simple majority – for things not directly tied to the budget, which very much includes Biden’s infrastructure bill.

“We should all be alarmed at how the budget reconciliation process is being used by both parties to stifle debate around the major issues facing our country today,” wrote Manchin, adding: “I simply do not believe budget reconciliation should replace regular order in the Senate. How is that good for the future of this nation? Senate Democrats must avoid the temptation to abandon our Republican colleagues on important national issues. Republicans, however, have a responsibility to stop saying no, and participate in finding real compromise with Democrats.”

Which is a very big deal. Especially when you consider that Senate Republicans – led by Mitch McConnell – have offered very little reason to believe they will come to the negotiating table with Biden.

“I’m going to fight them every step of the way, because I think this is the wrong prescription for America,” McConnell has said of the $2 trillion package.

If McConnell can keep his side in line – and Manchin continues to oppose using reconciliation to pass the infrastructure bill – then the legislation will stall. And in order to restart it, Biden (and Senate Democrats) will have to make major compromises – read: cuts – to the size of the proposal in order to get the 60 votes it would need to end filibuster and bring the measure to a final vote.

Biden, perhaps aware of this emerging political reality, seemed to open the door to compromise on the legislation in comments on Wednesday. “Debate is welcome. Compromise is inevitable. Changes are certain.” But in a sign to Republicans that he wouldn’t accept “no deal,” Biden added: “Here’s what we won’t be open to: We will not be open to doing nothing. Inaction is simply not an option.”

The lines about “compromise” and “debate” will, no doubt, be music to Manchin’s ears. (And Biden may have been aiming exactly at Manchin with his remarks.) The question is whether what Manchin wants – and what Biden says he is open to – is possible or achievable.

“Instead of fixating on eliminating the filibuster or shortcuting the legislative process through budget reconciliation, it is time we do our jobs,” Manchin writes. Which, yes, bipartisan compromise would be a good thing! Voters repeatedly tell pollsters that they want members of Congress to work together. And there’s no question that an infrastructure bill – or any bill – that passes with a large bipartisan vote would send a powerful message to a public wondering whether Washington will ever get along (or even if they should).

But all of that presumes that Republicans have any real interest in meeting Democrats halfway – on infrastructure, guns, voting rights or any other major issue facing the country.

Liberals insist that is not the case, that Manchin’s desire to return to the good old days of bipartisanship misses the fact that the current iteration of the GOP has zero incentive to make any sort of deal with Democrats.

“You have Republicans in the Congress who are moving very far to the right, who are not only ignoring what the American people want to see done,” Vermont Sen. Bernie Sanders told CNN’s Jake Tapper on Sunday. “They are ignoring what people in their own party want to be done.”

Manchin and Biden, who spent decades burnishing a Senate resume with loads of bipartisan credentials, still clearly believe that Republicans want compromise and will come to the table for, uh, the good of the country.

With Manchin’s announcement on the filibuster – and the reconciliation process – that theory will be put to the test. Compromising with Republicans on infrastructure – and everything else – will be the only way to get things done. If ever there was a moment in which bipartisanship is being put to the test in the modern Senate, this is that moment.