CNN  — 

He’s undecided on the nominee to head Health and Human Services. He’s not sure if he’ll back the No. 3 for the Justice Department, or the undersecretary of policy at the Pentagon.

He says maintaining the 60 vote-requirement to overcome a filibuster is a “red line” for him. And he’s made clear he’ll block advancing an infrastructure package on a party-line vote if Democrats don’t work with Republicans.

In a 50-50 Senate, with most members voting the party-line, Sen. Joe Manchin stands mostly alone: effectively holding veto power over many of President Joe Biden’s nominees and key elements of his agenda.

“It’s a shame,” Manchin, a Democrat from West Virginia, told CNN when asked about his newfound veto power. “There are 49 other Democrats. Please come forward.”

Manchin added: “It’s not good. It’s not a good place to be.”

But he offered this warning to his party as it tries to move legislation along straight party lines: “I’m going to make sure they don’t.”

Manchin, a rare Senate Democrat from a deep red state that former President Donald Trump won in a landslide, has long occupied the slot as the most likely swing vote among Senate Democrats, a position the 73-year-old has prized as he hosts senators from both parties at his boat that he lives on in Washington.

Manchin joined the Senate after literally shooting the Democrats’ climate change proposal in a 2010 campaign ad – and more recently joining Republicans in the Trump years on some of the most controversial matters, such as voting to confirm Brett Kavanaugh to the Supreme Court and William Barr as attorney general. Yet he also voted to convict Trump in both impeachment trials and voted against efforts to repeal the Affordable Care Act.

Manchin argues that he’s simply trying to ensure the two parties work together in a highly polarized political environment. His Democratic critics say he’s giving far too much deference to an opposition party eager to thwart the President’s agenda.

Now, under Biden, Manchin holds a unique perch: the Democrat the new President needs to get on board when votes are bound to come down strictly along party lines. And in private, Biden has encouraged Manchin to stick by his convictions, something he has already done as he’s frustrated some of the White House’s agenda.

Manchin’s opposition effectively sank the nomination of Neera Tanden to head the Office of Management and Budget, as he claimed her past tweets made her “too toxic.” And he sent Washington into nearly 12 hours of tension when he initially balked at a last-minute change on jobless benefits, a position Democrats feared could have sunk Biden’s $1.9 trillion Covid relief plan. Ultimately, he cut a deal and then backed the bill, which passed the Senate on a 50-49 vote with no GOP support.

Senate Democratic leaders say everybody in their caucus wields the same power. But Manchin is the one who wields it most frequently.

“Anybody in a 50-50 Senate is in a position to be able to do that,” said Sen. Debbie Stabenow, a Michigan Democrat and member of leadership. Asked if Manchin should be careful not to overreach, Stabenow: “I think all of us need to be careful about that.”

With the flood of attention he’s getting, some of his colleagues have no desire to talk about him, either.

“Here’s what I’m going to say about Manchin: Everybody asks me about him. Why don’t you just ask him?” said GOP Sen. Shelley Moore Capito, who also represents West Virginia. “Seriously. When you have a one-vote margin, everybody has a lot of influence. Talk to him about it.”

Indeed, several other members who caucus with the Democrats have the potential to be swing votes, such as Sens. Kyrsten Sinema of Arizona, Jon Tester of Montana and Angus King of Maine, while others facing difficult reelections could break from their party – like Sen. Maggie Hassan of New Hampshire, who joined with seven of her colleagues in voting against an effort to include a $15 federal minimum wage in the relief bill.

Undecided on key nominees

Yet few are generating as much attention from the White House as Manchin. With Biden’s nominee to be the Pentagon’s under secretary for policy, Colin Kahl, hanging by a thread, Defense Secretary Lloyd Austin jumped on the phone to lobby for Manchin’s support. The West Virginia Democrat also spoke with a former defense secretary, Bob Gates, and Kahl himself.

In his conversation with Manchin, Kahl expressed regret for his past tweets, and Manchin said they were “nowhere near as prolific” as Tanden’s.

“Tweets,” Manchin said, sounding exasperated. “I don’t know why people want to get on there. I really don’t know.”

Other nominees have come to woo Manchin as well – the latest being Xavier Becerra, who was nominated as secretary of Health and Human Services. With GOP opposition lining up, on the grounds Becerra is too liberal for the post, Manchin’s support could be decisive.

“Here’s the thing, I’ve been pretty differential on that, you follow me?” Manchin said of presidential nominees.

On Becerra, Manchin said: “His political background is different than mine. OK? I really appreciate his fight for the ACA because of all the people with preexisting conditions and 800,000 West Virginians would have lost, and he fought for that. But some of his other stances are different than mine. But you understand, it’s not his policy he’s pushing” if he were to be confirmed.

It remains to be seen whether any Republicans will back Becerra, with both GOP Sens. Lisa Murkowski of Alaska and Susan Collins of Maine signaling they were undecided and still having discussions with Becerra, and Sen. Mitt Romney of Utah telling CNN he planned to oppose him.

“He’s just out of the mainstream of his own party with regards to abortion and with regards to religious freedom,” Romney said of Becerra on Wednesday.

Other nominees could potentially hinge on winning all Democratic support, such as Vanita Gupta as the No. 3 at the Justice Department, though Manchin signaled he has yet to review her nomination.

For Biden and Senate Majority Leader Chuck Schumer, they recognize the significance of Manchin’s vote – something that became clear through the course of a hectic day of negotiations last Friday over the relief package. Schumer bluntly warned Manchin that supporting a Republican alternative on jobless benefits would kill the overall relief bill, while Biden engaged in more of a soft sell: encouraging him to vote his conscience.

“I wanted Donald Trump to succeed,” Manchin said in the interview. “I want Joe Biden to succeed. … But like he said, ‘Joe,’ he said, ‘I’ve never asked you to go against your convictions.’ And I told him, ‘I appreciate that, Mr. President, because I won’t.’ ”

Manchin vows to block party-line efforts

Indeed, as Democrats are trying to push forward a massive infrastructure package, Manchin has bluntly said he would block the effort if they try to advance the plan on a straight party-line vote and use a budget process called “reconciliation” – and don’t try to win over Republicans first. It’s the same process that Democrats used to pass the sweeping relief bill that cleared Congress on Wednesday.

Yet if Democrats don’t use the budget process, they’ll have to win at least 10 Republicans to break a filibuster, which requires 60 votes, something that could be a daunting task.

“It would be nice to find some common ground first,” Tester said of trying to win over Republicans before moving ahead on reconciliation. He added with a laugh: “But I don’t want to sound like Joe Manchin.”

Many Democrats over the weekend were heartened when Manchin signaled an openness to revising the filibuster, a tool he has long vowed to protect in the name of his predecessor, the late West Virginia Sen. Robert Byrd, a fierce defender of the Senate.

Manchin is suggesting he could be open to forcing senators to hold the floor – rather than the current situation, where a simple threat to filibuster is enough to slow down the body and force a vote at a 60-vote threshold.

But Manchin has made clear he won’t back away from the 60-vote threshold, telling CNN that it’s a “red line” for him. That means Democrats won’t have the votes to change the filibuster rules in order to allow 51 senators to advance legislation, even as a growing number in their party are demanding that monumental change in the face of GOP opposition to many of their big-ticket agenda items.

“The red line is having minority participation in your process,” Manchin said. “That’s the way we were designed. … I haven’t seen any reason to come off the 60.”

Manchin’s colleagues on both sides are watching him closely.

“I hope he sticks to his guns,” said Sen. Lindsey Graham, Republican of South Carolina. “I’m sure Robert Byrd would want him to. And the people of West Virginia. We cease to be the Senate if that goes away.”

CNN’s Olanma Mang and Ted Barrett contributed to this report.