CNN  — 

Sen. Joe Manchin, the most pivotal swing vote in the Senate, indicated on Monday that a significant amount of work remains to be done to earn his support for President Joe Biden’s sweeping social safety net expansion, potentially delivering a fatal blow to Democratic leaders’ hopes of getting the bill passed in the Senate before Christmas.

In a critical moment for the party’s agenda, Manchin spoke with Biden on Monday afternoon as the President tries to secure his support for the plan. But the senator, whose vote is essential to its passage, is raising serious concerns, citing issues with the proposal’s reliance on temporary programs and renewing long-standing concerns over inflation that only intensified after a report last week showed a key inflation measure surging to a 39-year high.

A spokesperson for the senator said in a statement Monday evening that Manchin and the President “had a productive conversation,” and “will continue to talk over the coming days.”

The concerns that Manchin is raising, however, suggest that he needs to see wholesale changes to the bill, a process that could take weeks or even potentially months if he holds firm. But Democratic leaders have been racing to get the long-awaited bill done before Christmas even though the final proposal is still being written and vetted by the Senate parliamentarian

Manchin said Monday evening that he’s not walking away from talks. When asked if he thinks the bill can get done this year, he said: “anything is possible” and he’s still “engaged” in discussions.

But the West Virginia Democrat, who has already accused his party of using “budget gimmicks” in drafting the plan, is now objecting to the way that Democrats are structuring the legislation, which he argues hides the true cost by relying on temporary spending that will likely become permanent.

In a key warning sign, Manchin told CNN the bill should not rely on temporary spending that lawmakers will face pressure to extend. “I don’t think that’s a fair evaluation of saying we are going to spend X amount of dollars but then we are going to have to depend on coming back and finding more money … I’m concerned about paying down debt too,” he said.

Manchin said the bill should be “within the limits of what we can afford” — and argued that lawmakers should evaluate how much it would cost to extend temporary programs in the bill for 10 years to be “transparent” about the actual price tag of the bill. Extending the programs would drive up price tag.

The Congressional Budget Office recently released a revised score for the plan based on a Republican request to see how much the bill would cost if a series of provisions were extended for a full 10 years. The CBO estimated that the version of the legislation without sunsets would “increase the deficit by $3 trillion over 2022 to 2031.” The current bill, known as the Build Back Better Act, does not include such extensions to the provisions.

Manchin said that the new CBO report was “very sobering” and that he’s been “very concerned” about the price tag. While many Democrats have dismissed and downplayed the CBO report, Manchin defended the estimate, another warning sign for his party.

Manchin argued that the office is “non-partisan,” saying, “they’re going to give us the facts whether we like it or not.”

He added, “you might as well look at the whole ball of wax if you will,” when asked if he doesn’t believe that temporary programs will stay temporary.

Ahead of his conversation with Biden, Manchin said that he planned to talk with the President about “exactly what happened on Friday with the CBO score and inflation reports.”

The senator continued to outline his serious concerns over rising inflation and the fiscal impact of the proposed $1.9 trillion plan to deliver aid to families, expand access to health care and deal with climate change.

“Inflation is real. It’s not transitory,” he said. “It’s alarming. It’s going up, not down. I think that should be something we are concerned about.”

This story has been updated with additional developments Monday.

CNN’s Anneken Tappe contributed to this report.