Civil rights leaders who met with Manchin said they pressed him on the filibuster as a Jim Crow relic that needed to be discarded to secure fundamental democratic rights for Americans. Instead, they were struck by his faith that he could persuade Republicans to back a bill on the issue.

“He said he thought he could do it,” said Campbell, who participated in numerous meetings with senators, including Manchin.

Meanwhile, Sinema was amping up her own warnings — penning The Post op-ed reaffirming her long-standing support for the existing 60-vote filibuster — while she was also in the thick of another legislative battle, one calibrated around her belief in the power of bipartisanship. That effort — a $1 trillion infrastructure deal — was consummated a month later, and Sinema quickly touted it as proof that Republicans and Democrats could work together in a 60-vote Senate.

“How many times have we heard that bipartisanship isn’t possible anymore? Or that important policy can only happen on a party line?” she said at a White House signing event in November, as Schumer, Biden and other party leaders looked on. “Our legislation proves the opposite.”

But it was becoming clear that there was little bipartisanship to be had on voting rights, particularly on a bill as expansive as the one Democrats sought to pass Wednesday. Not only had Republicans voted consistently to oppose even debating the Democratic bills, efforts to try to forge any sort of compromise were being rebuffed.

Senate Minority Leader Mitch McConnell (R-Ky.) successfully rallied his Republican conference behind a position that was at odds with the plain text of the Constitution but fit the mood of his members — that Congress simply had no role in telling states how to run their elections.

“This party-line push has never been about securing citizens’ rights — it’s about expanding politicians’ power,” McConnell said Wednesday.

Through the fall, Manchin met with several Republican senators, including McConnell, to pitch them on potential cooperation. Other senators with solid bipartisan relationships who had become convinced of the need to drastic action, including Tim Kaine (D-Va.) and Angus King (I-Maine), also worked back channels to little avail. Even the Republicans most inclined toward cooperation, such as Susan Collins (Maine) and Rob Portman (Ohio), were interested in a much narrower bill that would steer clear of new federal voting standards.

Kaine said Democrats were “delivered a message about the facts of life” from a GOP senator he declined to name.

“[The senator] just said, ‘Look, we love Mitch McConnell. . . . He lets us do whatever we want. He may not agree with it. He may not vote for it, but he lets us deal with everyone,” Kaine said. “ ‘He has two red lines, voting rights and campaign finance. And he’s told us absolutely plainly you cannot work with Democrats on either of those issues.’ ”

Advocacy groups, meanwhile, pushed Biden over the summer to take a more aggressive stance and make a more concerted push to bring Manchin and Sinema along. But, according to people in touch with the White House and key congressional offices, top Democrats had concluded that the party’s economic agenda — a precarious effort to pass the bipartisan infrastructure bill and a Democrats-only domestic policy bill in tandem — was the priority.

Biden said as much in a CNN town hall in October, explaining that a showdown on the Senate rules would have to wait.

“Here’s the deal — if, in fact, I get myself into at this moment the debate on the filibuster, I lose at least three votes right now to get what I have to get done on the economic side of the equation, on the foreign policy side of the equation,” he said.

At the same time, however, the former six-term senator moved closer to openly endorsing rules changes, saying he believed Democrats would have to “fundamentally alter the filibuster.”

Through the summer and fall, frustration rose on Capitol Hill and among advocates, with many pushing Biden and Harris, whom he had deputized to advance the issue, to launch a more vigorous campaign for action. Other fissures developed, with some wondering whether the initial Democratic bill — a grab bag of proposals almost entirely written before the 2020 election — was ill-suited to respond to the present threats to democracy.

It was only in mid-December, after Manchin expressed fresh reservations about the domestic policy bill, effectively pushing any chance of its passage well into 2022, that Democratic leaders clearly pivoted to the voting rights fight. But by the time debate began last week, many states were only months away from holding their midterm primaries, and even some Democratic elections officials worried about implementing any new federal mandates this year.

Meanwhile, Manchin — the only statewide-elected Democrat in a state that voted for Trump by 39 points — remained largely immune to political pressure even as he quietly discussed potential rules changes with Democratic colleagues. Sinema was even less receptive to the discussion, according to multiple Democrats who attempted to persuade her, pointing in private conversations to her infrastructure triumph, as well as her experiences as a state legislator in Arizona, as proof that bipartisan cooperation is possible.

In the end, neither senator budged. Manchin said Wednesday that senators “wasted a year behind the scenes, talking through each other, around each other, but not to each other.” Sinema said in a statement that the rules change would “deepen our divisions and risk repeated radical reversals in federal policy” — echoing her words in the June op-ed.

The fight did pay some dividends for party leaders, though it may take years before they can fully collect. Dozens of Democrats who had been wary of eroding the filibuster — including several facing tough reelection fights, such as Sens. Maggie Hassan (N.H.) and Mark Kelly (Ariz.), where the filibuster threatens to be a polarizing issue — ultimately backed Schumer.

“If I would have said ‘change the rules’ back in March, I might have had half my caucus. But as people focused on voting rights, it grew and grew and grew,” Schumer said, adding that the battle got voters to engage and, in turn, pressure lawmakers.

“When people feel so much heat, they usually move in a direction to relieve that heat,” he said.

No senators felt more heat than Manchin and Sinema, who have faced fierce political backlash from inside their own party. Advocates said that is likely to deter future Democratic candidates from adopting similar institutionalist views, thus making future action more likely.

The debate over the filibuster “is now a top-of-mind issue in Senate races,” Muller said. “I don’t think there’s going to be a Senate candidate out there who is not going to have to take a position on it, and that is a big change from what it was even last cycle.”

Schumer disputed that Wednesday’s failure came at a political cost. Sidestepping the fight in the Senate, he said, would have disillusioned even more Democratic voters — “particularly our core constituencies,” he said. “They know you don’t succeed in a linear line. They know you fail when you don’t fight.”

When pressed on what the failed vote accomplished, many of those involved in the fight have reached for moral and historical comparisons.

But some said Democrats also had a political obligation — to fight on behalf of the minority voters who had sent Biden and Harris to the White House and handed Democrats their congressional majorities and now faced new hardships at the ballot box.

“They are targeting people and trying to exclude them because those people are assumed to be loyal to us. So are we loyal to them?” said Kaine, who was involved in a months-long attempt to convince the holdout Democrats. “If they’re being punished and hurt because they’re assumed to be loyal to us, like, how can we not have this battle?”