Paid leave has become a significant sticking point in the effort to pass President Joe Biden’s Build Back Better legislation, a $1.75 trillion social welfare and green energy bill.

Senate Democrats set a Christmas deadline for passing the bill, but it will stall without the cooperation of all 50 Democrats, including Sen. Joe Manchin of West Virginia.

Manchin hasn’t endorsed the bill yet, and one reason is the bill includes a provision to provide four weeks of paid leave for those who need to take care of a baby or deal with an illness.

House Democrats already reluctantly agreed to whittle down the paid leave benefit from 12 weeks to win over Manchin. But Manchin wants it out entirely.

“I don’t think it belongs in the bill,” Manchin told CNN last month.

Manchin’s opposition now threatens passage of the entire bill and is infuriating liberals who believe paid leave is one of the most critical provisions in the legislation.

Hundreds of New York community and grassroots activists advocating for paid family leave wrote to Senate Majority Leader Chuck Schumer this week, warning him against removing the provision from the legislation.

“Simply put, on your watch, paid leave must not be left on the cutting room floor,” the group wrote to the New York Democrat. “We believe that every working person in New York and beyond deserves time to care for themselves and their loved ones regardless of where they live or work."

Democrats hope to tout paid leave among the new array of government services and subsidies Build Back Better would provide.

It was a key campaign promise by Biden and congressional Democrats, and polling shows it is very popular, particularly with younger voters. A Morning Consult/Politico tracking poll released last month found the majority of registered voters support at least four weeks of paid family and medical leave.

“We want to see paid leave in the bill,” Schumer said this week.

Speaker Nancy Pelosi and other House Democrats fought to keep paid leave in the legislation. They scrapped a plan to provide 12 weeks of paid family leave after Manchin warned he wouldn’t support it.

Pelosi then decided to include four weeks of paid leave in the House version, which passed late last month.

Democrats argue the United States must follow the lead of most other wealthy nations that provide paid leave.

Canada, for example, provides up to 35 weeks of leave at a rate of 55% of weekly insurable earnings.

The paid leave provision in the House bill would provide partial pay depending on a person’s earnings for someone who needs time off for a new baby, to recover from an illness, or to care for a sick family member.

Advocates and fellow Democrats are stepping up pressure on Manchin to support the provision.

A paid leave advocacy group, Paid Leave for All, produced a new poll showing that 80% of West Virginia voters back paid family leave for anyone dealing with an illness, while 75% back paid leave for new parents.

Senate Budget Committee Chairman Bernie Sanders, a Vermont independent and a socialist, has insisted the paid leave provision remain in the measure, publicly pressuring Manchin, but not by name.

“We are the only major country on the planet that doesn't guarantee a single day of paid family and medical leave,” Sanders tweeted. “You have a baby in America today, and you're low-income, you're back at work in a day or two. That is barbaric.”

Manchin said he is not opposed to creating a federal paid leave program but doesn’t want Democrats to pass it without GOP support because it could be reversed when Republicans retake the majority at some point.

The provision would cost $200 billion, according to the nonpartisan Congressional Budget Office.

Manchin said he’d begun talks with centrist Republicans, hoping to generate momentum for a separate measure providing paid leave.

“We can do it in a bipartisan way,” Manchin told CNN. “We can make sure it’s lasting.”

But most Republicans are unlikely to support it.

Republicans argue many private companies already provide paid leave and that a federal mandate would increase the cost to employers while decreasing employment.