Top Biden administration officials were laboring Wednesday to try to reach a last-minute deal for wartime aid for Ukraine by agreeing to Senate Republican demands to bolster U.S.-Mexico border policies, with urgency setting in as Congress prepared to depart Washington with the impasse unresolved.

The White House was racing to lock in a deal in principle with key Senate negotiators, which would allow them to work on the text of legislation through the holiday break, according to two people familiar with the plans who demanded anonymity to discuss them.

As details of the plan emerged, advocates for immigrants and members of President Joe Biden’s own Democratic Party fretted about the policies under discussion. Some demonstrated at the Capitol, warning of a return to the hardline border and immigration policies of the Trump era.

Congress has little time to reach an agreement on Biden's $110 billion request for Ukraine, Israel and other national security needs that Republicans are holding up to demand changes to border policy. While White House officials and key Senate negotiators appeared to be narrowing on a list of priorities to tighten the U.S.-Mexico border and remove some recent migrant arrivals already in the U.S., Senate Republicans said that not enough progress had been made to justify staying in Washington beyond Thursday.

Ukrainian President Volodymyr Zelenskyy visited Washington this week to implore lawmakers for support, but lawmakers were still ready to leave for weeks with one of the U.S.'s key international commitments — helping halt Russian President Vladimir Putin's invasion into Ukraine — seriously in doubt. Also left hanging would be a deal on one of the most unwieldy issues in American politics: immigration and border security.

“This is difficult, very difficult,” Senate Majority Leader Chuck Schumer, D-N.Y., said in a speech on the Senate floor Wednesday. “But we’re sent here to do difficult things.”

Among the proposals being seriously discussed, according to several people familiar with the private talks, are plans to allow Homeland Security officials to stop migrants from applying for asylum at the U.S. southern border if the number of total crossings exceeds daily capacity of roughly 5,000. Some one-day totals this year have exceeded 10,000.

Also under discussion are proposals to detain people claiming asylum at the border, including families with children, potentially with electronic monitoring systems.

Negotiators are also eyeing ways to allow authorities to quickly remove migrants who have been in the United States for less than two years, even if they are far from the border. But those removals would only extend to people who either have not claimed asylum or were not approved to enter the asylum system, according to one of the people briefed on the negotiations.

The policies resemble ones that President Donald Trump's Republican administration tried to implement to cut border crossings, but many of them were successfully challenged in court. If Congress were to make them law, it would give immigration advocates very little legal ground to challenge the restrictions for those seeking asylum.

Advocates for immigrants, who are planning demonstrations across the Capitol on Wednesday, warned of a return to anti-immigrant policies and questioned whether they would even address problems at the border.

“I never would have imagined that in a moment where we have a Democratic Senate and a Democratic White House we are coming to the table and proposing some of the most draconian immigration policies that there have ever been,” said Maribel Hernández Rivera, American Civil Liberties Union director of policy and government affairs.

The Senate negotiations had also found some agreement on raising the threshold for people to claim asylum in initial credible fear screenings.

Senate Republicans discussed the White House's proposal at a lunchtime meeting and expressed some optimism that Biden's administration was directly involved in shaping the legislation. But many senators said there was simply not enough time to iron out an agreement.

“Nobody’s written anything up. These are all concepts right now,” said Sen. John Thune, the no. 2 Senate Republican, adding, “The deal has not come together."

Even if the Senate stayed in Washington to pass the proposals, House Speaker Mike Johnson of Louisiana, a Republican, would also need to push the legislation through his chamber, where there will likely be opposition from both parties. Hard-line conservatives complain the Senate proposals do not go far enough, while progressive Democrats and Hispanic lawmakers are opposed to cutting off access to asylum.

At a press conference in front of the Capitol, leaders of the Congressional Progressive Caucus and Congressional Hispanic Caucus vowed to oppose the policies under consideration. They also said that Latino lawmakers should have been central to the negotiations.

“(Biden) campaigned on restoring the soul of the nation and holding firm to our democratic values and the principles of our founding fathers. And that includes defending our asylum system and respectful treatment of refugees," said Sen. Alex Padilla, D-Calif.

He called it “unconscionable” for the Democratic president to make concessions on border policy without gaining policies that benefit immigrants.

White House press secretary Karine Jean-Pierre said the administration was “encouraged” by progress in the negotiations and stressed that any final product has to be a “bipartisan compromise.” She declined to address criticism from advocates that the provisions under discussion could be more draconian than that of Biden’s predecessor, Donald Trump.

In the Capitol, the senators who have been negotiating the border package urged congressional leaders to call lawmakers back to Washington next week, hoping that they could use this week's momentum to push through the package.

But their colleagues warned that having the Senate pass the package would be futile unless the House was ready to move quickly.

“It’ll be a piñata out there that people take potshots at for the next couple of weeks,” said Sen. John Cornyn, R-Tex.