WASHINGTON — As the Senate wrapped up its work for the year, Sen. Michael Bennet took to the floor of the nearly empty chamber and made a late-night plea for Congress to redouble support for Ukraine: “Understand the stakes at this moment.”

It was the third time in recent months the Colorado Democrat has kept the Senate working late by holding up unrelated legislation in a bid to cajole lawmakers to approve tens of billions of dollars in weaponry and economic aid for Ukraine. During a nearly hour-long, emotional speech, he called on senators to see the nearly 2-year-old conflict as a defining clash of authoritarianism against democracy and implored them to consider what it means for Ukrainians to fight "on that freezing front line and not know whether we're going to come through with the ammunition.”

Yet Congress broke for the holidays and is not expected to return for two weeks while continued aid for Ukraine has nearly been exhausted. The Biden administration is planning to send one more aid package before the new year, but says it will be the last unless Congress approves more money.

With support slipping in Congress even as conflicts and unrest rattle global security, the United States is once again struggling to assert its role in the world. Under the influence of Donald Trump, the former president who is now the Republican Party front-runner, GOP lawmakers have increasingly taken a skeptical stance toward U.S. involvement abroad, particularly when it comes to Ukraine.

Leaders of traditional allies Britain and France have implored Western nations to continue their robust support, but Russia’s President Vladimir Putin is emboldened and building up resources for a fresh effort as the war heads towards its third year.

Bolstering Ukraine's defense used to be celebrated in the Capitol as one of a few remaining bipartisan causes. But now the fate of roughly $61 billion in funding is tied to delicate policy negotiations on Capitol Hill over border and immigration changes. And in the last year, lawmakers have had to mount painstaking, round-the-clock efforts to pass even legislation that maintains basic functions of the U.S. government.

Still, congressional leaders are trying to rally members to address global challenges they say are among the most difficult in decades: the largest land invasion of a European nation since World War II, a war between Israel and Hamas, unrest and economic calamity driving historic levels of migration and China asserting itself as a superpower.

In the Senate, both Democratic and Republican leaders have cast the $110 billion aid package, which is attempting to address all those issues, as a potential turning point for democracy around the world. Senate Majority Leader Chuck Schumer told reporters last week that “history will look back if we don’t support our ally in Ukraine.”

In a year-end speech, Senate Republican Leader Mitch McConnell said: “From South Texas to Southeast Asia and from the Black Sea to the Red Sea, it is an historically challenging and consequential time to protect America’s interests, our allies and our own people.”

The Republican leader, a key supporter of Ukraine aid, has tried for months to build support in his party for Ukraine. But after a $6 billion military and civilian aid package for Ukraine collapsed in October, McConnell began telling top White House officials that any funding would need to be paired with border policy changes.

The White House deliberately stayed out of the negotiations until senior officials felt the time was right to do so. But senior Republicans involved in the border talks believe the administration stepped in too late, ultimately delaying the prospects of additional Ukraine aid getting approved until the new year.

Senate negotiators have had to navigate both the explosive politics of border policy as well as one of the most complex areas of American law.

“This is a tightrope, but we are still on it,” said Sen. Chris Murphy, the lead Democratic negotiator.

The White House's strategy of including Republican priorities such as Israel aid and border security in the package has also raised several thorny issues for Democrats.

Progressive lawmakers, critical of Israel's campaign into Gaza that has killed thousands of civilians, have called for humanitarian conditions to be placed on the money for Israel. And Latino Democrats in both the Senate and House have also been critical of restrictions on asylum claims.

Any package also faces deep uncertainty in the House, where Republican Speaker Mike Johnson holds tenuous control of the closely divided chamber. Before becoming speaker in October, Johnson had repeatedly voted against aid for Ukraine, but he has surprised many by offering support for Ukrainian President Volodymyr Zelenskyy and saying he wants to find a way to approve the aid.

But Trump’s allies in the House have repeatedly tried to stop the U.S. from sending more aid to Ukraine.

As the border and immigration talks drag forward in the Senate, Johnson has pushed for sweeping measures. On social media, he has called for “transformational change to secure the border,” and pointed to a hardline bill that passed the House on a party-line vote.

As senators left Washington, they still sought to assure Ukrainians that American help was on its way. White House staff and Senate negotiations planned to work on drafting border legislation for the next two weeks in hopes that it would be ready for action when Congress returns.

Schumer told the Associated Press he was “hopeful,” but “I wouldn’t go so far as to say confident yet.” He sought to put the pressure on Republicans, saying they needed to be ready to compromise.

Yet Sen. Roger Wicker, a Mississippi Republican who is a Ukraine supporter, expressed confidence that Congress would act. He alluded to the words of former British Prime Minister Winston Churchill, another European leader who eventually elicited robust support from the U.S. to repel an invasion.

“Americans will always do the right thing,” Wicker said. “After they’ve exhausted every other alternative.”