By Richard Cowan and Leah Douglas

WASHINGTON Democrats in the U.S. Congress expressed frustration with President Joe Biden's willingness to engage with Republicans demanding tougher work requirements for food aid recipients as part of any deal to raise the nation's debt ceiling.

They have stopped short of threatening to block such moves, as talks on lifting the federal government's $31.4 trillion borrowing limit shifted into a bilateral format between Democrat Biden, Republican House Speaker Kevin McCarthy and their staffs.

If Biden and McCarthy reach a deal, possibly as soon as Sunday, Congress could struggle to get enough votes for passage ahead of a June.

Some hardline Republicans may push back against any increase in the debt ceiling, while some progressive Democrats voiced opposition to the work limits after spending months calling for a "clean" hike without conditions.

Liberal Democrats, including Senator Raphael Warnock and Representative Ro Khanna, put Biden on notice that they do not support more stringent requirements to existing law.

Khanna, asked whether revisions would prompt him to vote to torpedo a deal, said: "It would be a strong consideration."

Republicans have called for saving $120 billion by expanding work requirements to qualify for food aid, monetary help for poor families and other assistance. Biden on Wednesday reiterated his opposition to imposing new requirements the Medicaid program for low-income Americans.

He added that there could be a "few" changes in current law but none "of any consequence."

Those assurances did not soothe Democrats, as negotiations kicked into high-gear over spending and the urgent need to raise the borrowing limit.

Warnock accused Republicans of "using poor people as pawns" in negotiations, saying their proposal "presupposes that poor people are in some way morally deficient. People want to work. And some people can't."

Warnock is a member of the Senate Agriculture Committee which, writes the farm bill that funds the Supplemental Nutrition Assistance Program, commonly known as food stamps.

A Republican bill that passed the House in late April would place more work demands for SNAP on adults without disabilities or dependents up to age 56, instead of the current cutoff of 49.

Massachusetts's Jim McGovern, a House Democrat known for his anti-hunger work, said: "I will not support anything that screws poor people -- period."

The Republican-controlled House Ways and Means Committee describes the steps as "common-sense work requirements to help lift families out of poverty and revitalize the American workforce."

"Why wouldn't he want to help people get out of poverty?" McCarthy told reporters.

A USDA spokesperson said expanded work requirements for SNAP would be especially harmful to those "experiencing homelessness or people affected by local economic conditions like the closure of a major local employer."

USDA estimates that an additional 1 million low-income older adults would be subject to SNAP time limits and, as a result, could lose vital food benefits.

Eric Mitchell, executive director of the non-profit Alliance to End Hunger, called such requirements "punitive and ineffective" against people who face obstacles to employment or community service.

(Reporting by Richard Cowan and Leah Douglas; Editing by Scott Malone and Aurora Ellis)