By Manu Raju, Ted Barrett, Phil Mattingly and Lauren Fox, CNN

(CNN) -- It was Friday afternoon, and President Joe Biden was on the phone with Sen. Joe Manchin, the conservative West Virginia Democrat who felt blindsided by his party leaders and now was at risk of defecting and effectively torpedoing a central pillar of the White House's domestic agenda.

Just a couple hours earlier as he walked onto the floor ahead of votes on Biden's $1.9 trillion Covid relief plan, Manchin was stunned about the last-minute dealings of his party's leaders with the backing of the White House. For the first time, he learned that Democrats were seeking to advance a new plan on jobless benefits that would allow people to ensure their first $10,200 was tax free.

Manchin told colleagues he knew nothing about that tax provision. Yet Democratic leaders thought that Manchin was on board since their rank-and-file senators had reached out to him and reported back to party bosses that he was with them. They'd told the White House things were in good shape.

But Manchin was not on board, making clear he was not going to be jammed by his party leaders into accepting something he didn't like, according to sources who talked him, underscoring the power of an individual senator to derail the new President's agenda in the 50-50 Senate.

Asked why Democratic leaders' dispute with Manchin wasn't resolved ahead of time, Michigan Sen. Debbie Stabenow said Saturday: "We thought it was."

The resulting confusion and chaos -- and miscommunication between Senate Majority Leader Chuck Schumer and his most important swing vote -- put Washington on edge, threatened to blow up a delicately negotiated compromise and forced Democrats to leave a vote open longer than any in modern history -- nearly 12 hours -- as they engaged in a furious lobbying campaign and horse-trading to get Manchin on board.

Privately, about half a dozen Democratic senators were engaged in constant discussions with Manchin, hoping to find a way to win him back -- all as Ohio Republican Sen. Rob Portman was speaking with Manchin multiple times an hour about his own alternative that Manchin supported but that Democratic leaders wanted to defeat.

"I was trying to be the catcher in the rye. I was trying to keep him talking to the leadership," said Sen. Angus King, an independent from Maine who said he was speaking with Manchin "constantly" on Friday. "My goal was to keep him talking and not let it fall apart."

For Biden, though, the approach amounted to a soft-sell. He was deliberately careful not to add pressure to the situation, instead choosing to give Manchin space and listen to his concerns, a source with knowledge of the discussion said. But Biden did underscore one overarching point: how crucial it was to reach an agreement to get the bill toward the finish line given the urgency of the current economic and public health crisis. Manchin, two sources said, was urged by the President to do what he thought was right -- in essence to vote his conscience.

It was a reflection of a relationship that multiple sources said has been in a solid place since Biden took office -- Manchin of the mind that Biden is an honest broker, and Biden cognizant of the fact Manchin is his own senator and doesn't take kindly to being jammed.

Manchin, who characterized his Friday conversation with Biden as "good," said Saturday the talks "took longer than it should have." But he added: "We got it done, and we got a better deal."

"Let's just say we had some projects that lasted 12 hours," said Senate Majority Whip Dick Durbin. "It took a little longer than we thought.

Manchin had privately signed off on GOP plan

Days prior, Manchin had committed to backing an alternative plan by Portman, who proposed extending jobless benefits to $300 a week through July, reducing the benefit by $100 in the bill that passed the House.

But Democrats, fearful that the Portman plan would pass, worked privately behind the scenes to move forward with their own proposal to head off the Portman plan. The new Democratic plan would also reduce the benefits to $300 but extend them through September. And in order to head off backlash from the left by simply slashing back on the benefits, they added a sweetener: Ensuring that the first $10,200 would not be taxed.

By mid-morning Friday, Democratic leaders were feeling confident with where they were heading. They believed Manchin had signed off on the proposal, after his conversations with a fellow swing vote, Democratic Sen. Kyrsten Sinema of Arizona, and Delaware Sen. Tom Carper, the lead sponsor of the plan. But Manchin said Friday morning he had no knowledge of the tax provision. It's unclear how the message was lost in translation.

"I never heard about this at all. This is the first time I've ever heard about it, that morning. I said, 'Wait a minute,'" Manchin told reporters Saturday about the provision.

And he bluntly told his colleagues Friday he would not vote for the plan unless there were changes, prompting a day-long lobbying effort and leaving many of his colleagues perplexed.

Asked by CNN why he didn't resolve his differences with Manchin on the front-end, Schumer suggested he was surprised by the pushback by his West Virginia colleague.

"People have new differences all the time," Schumer said Saturday after the bill's passage. He added: "That eight hours is meaningless compared to the relief that the American people are going to get. And if it helped us get to that, great."

The failure to sort out their differences on the front-end surprised many Democrats because the two men have a frank and blunt relationship.

"I truly love all of my caucus," Schumer said. Even Manchin? "Yes. Absolutely," the New York Democrat said.

But many were nervous that the whole effort could collapse.

On Friday as Schumer, Carper and others were scrambling to fix the Manchin problem, Carper sat alone on a couch in the private Senate foyer. He was hunched over with his head almost between his knees and a phone held tightly to his ear. He looked both pained by what was happening and intent on fixing it. He later told a Republican colleague things were looking grim.

"We're stuck," Carper told Texas Sen. John Cornyn in the halls of the Senate.

"It was frustrating," Stabenow said when asked about the nearly 12 hours that the Senate was in gridlock. "We were trying to figure a way forward, which we did. I always felt that was possible. I always felt we would."

Day-long horsetrading and arm-twisting

Manchin and Democratic leaders were frantically trading proposals back and forth, multiple sources familiar with the talks said. Manchin, according to one source familiar with the matter, wanted the amendment broken up into two so that the Senate could vote separately on the weekly unemployment benefit of $300 and the tax-free component.

Without the two items packaged together, however, Democratic leaders knew the tax-free piece would fail. Leadership tried to impress upon Manchin multiple times that what the senior senator from West Virginia was asking for would not pass the House, where Speaker Nancy Pelosi had her own narrow margin and a group of progressives was already upset that the $15 minimum wage had been stripped from the Senate bill.

"It was pretty clear that there was an enormous amount of pressure," said South Dakota Sen. John Thune, the No. 2 Republican. "In a situation in the Senate with narrow margins, every man is a king and every woman is a queen."

The White House was involved and consulted throughout, with officials working through the numbers of the various proposals that were traded back and forth, officials said. But they were also aware that this was a matter being negotiated by key Senate Democrats -- and made a point of being available, but not overbearing, the officials said. Biden was available, but would only end up making one call to Manchin. He did, however, stay in touch with Schumer throughout.

A senior administration official specifically pointed to Carper and Sens. Ron Wyden of Oregon, Jon Tester of Montana and Mark Warner of Virginia, along with Schumer, as individuals who "deserve a huge amount of credit" for working through the final compromise.

The primary focus for the White House throughout the hours of talks was ensuring any agreement did not change the overall contours of the rescue plan, the official said. That baseline, which was the driving force for the creation of the plan itself, was ensuring the help "goes to the people who really need the help and being realistic about how long it will take the economy to recover," the official said.

The final agreement with Manchin, White House officials determined, would still accomplish those goals. Biden agreed.

"The end result is essentially about the same, and so I don't think any of the compromises have in any way fundamentally altered the essence of what I put in the bill in the first place," Biden told reporters Saturday after the vote.

Ultimately, they all signed off on a plan to cap the eligibility of the non-taxable component to households with an annual income under $150,000 and extend the $300 in weekly benefits until September 6.

After the deal was struck, senators were ready to vote. But it wasn't until an hour before midnight when the Senate would begin voting in earnest on the flood of amendments that Republicans proposed, forcing senators to pull an all-nighter as Democrats rejected GOP calls to adjourn until Saturday morning.

Sen. Jim Inhofe, an 86-year-old Oklahoma Republican who told CNN he suffered a concussion in the last week after falling on ice, said he stayed up all night until the final vote to pass the bill shortly after noon on Saturday. But it wasn't easy.

"It made for a very difficult night," Inhofe said of his concussion.

On Saturday, as they were waiting for the final passage vote to start, Manchin walked all the way over to Portman's desk and the two talked for several minutes in a very friendly and animated way.

As Manchin was about to leave they gave a little bit of a Covid-inspired hug, shoving their shoulders together, and Portman very quickly put his arm around Manchin's back.

Asked about the exchange later, Portman said: "We have to continue to work together and I appreciate that he kept his word and stuck to my proposal even though he was strongly persuaded not to. I looked at him one point like 12 hours ago and said, 'Joe, your arm looks kind of mangled. Was it twisted?'"