WASHINGTON (AP) — As Congress returns to session this week, lawmakers will be trying to forge an agreement on sending a new round of wartime assistance to Ukraine. But to succeed, they will have to find agreement on an issue that has confounded them for decades.

Republicans in both chambers of Congress have made clear that they will not support additional aid for Ukraine unless it is paired with border security measures to help manage the influx of migrants at the U.S.-Mexico border. Their demand has injected one of the most contentious issues in American politics into a foreign policy debate that was already difficult.

Time is short for a deal.

A small, bipartisan group in the Senate is taking the lead and working to find a narrow compromise that can overcome a likely filibuster by winning 60 votes. But even if they can reach a modest agreement, there is no guarantee it would pass the House, where Republicans are insisting on wholesale changes to U.S. border and immigration policies.

Republicans hope that Democrats will feel political pressure to accept some of their border proposals after illegal crossings topped a daily average of more than 8,000 earlier this fall. President Joe Biden, who is running for reelection next year, has faced pressure even from fellow Democrats over the migrant flow.

No matter what, finding compromise will be exceedingly difficult. As they left for Thanksgiving break, Senate negotiators said they were still far apart.

A look at some of the issues under discussion and why they have proved so difficult to resolve:

Asylum and humanitarian parole

Changing the asylum system for migrants is a top priority for Republicans. They want to make it more difficult for asylum-seekers to prove in initial interviews that they have a credible fear of political, religious or racial persecution in their home country before advancing toward asylum in the United States.

Republicans in the House have passed legislation that would detain families at the border, require migrants to make the asylum claim at an official port of entry and either detain them or require them to remain outside the U.S. while their case is processed.

U.S. and international law give migrants the right to seek safety from persecution, but the number of people applying for asylum in the U.S. has reached historic highs. Critics say many people take advantage of the system to live and work in the U.S. while they wait for their asylum claims to be processed in court.

Arizona Sen. Kyrsten Sinema, an independent who is part of the Senate negotiations, said in an Arizona radio interview that one of lawmakers’ goals is to ensure that “those who are here seeking asylum have an actual claim to asylum.”

Compromise is far from certain. Many Democrats are wary of making it harder to flee persecution, and the details of each policy shift are contentious.

Hardline conservatives in the House, already unlikely to support further Ukraine aid, have also signaled they won’t accept policy changes that deviate much from a bill passed in May that would have remade the U.S. immigration system. Their stance means at least some support from House Democrats will be needed to pass any agreement — no easy task.

Some progressives have already said they will oppose any Republican-led changes to immigration policy.

“The cruel, inhumane, and unworkable solutions offered by Republicans will only create more disorder and confusion at the border,” said Democratic Rep. Pramila Jayapal of Washington, chair of the Congressional Progressive Caucus.

Infrastructure and enforcement

Lawmakers may find it easier to reach consensus on other areas of border policy, particularly when it comes to border staffing and enforcement.

Negotiators have looked at steps that could be taken to reinforce existing infrastructure at the border, including hiring and boosting pay for border patrol officers and improving technology. One proposal advanced by a bipartisan group of senators would call for hiring of more border patrol agents, raising their pay and ensuring they receive overtime.

Biden has shown a willingness to accept tougher enforcement measures, recently resuming deportation of migrants to Venezuela and waiving federal laws to allow for the construction of border wall that began under then-President Donald Trump. The White House also wants to install new imaging technology at ports of entry that would allow authorities to quickly scan vehicles for illegal imports, including fentanyl.

Republicans say that is not enough. They want more robust improvements, including more expansive construction of a border wall.

What Biden is asking for

Biden’s emergency request to Congress included aid for Ukraine, Israel and other U.S. allies, along with $14 billion to bolster the immigration system and border security. Money would go toward hiring more border patrol agents, immigration judges and asylum officers. It’s part of Biden’s strategy of trying to simultaneously turn away from Trump’s hard-line policies but adapt to the realities of crossings at the U.S.-Mexico border.

Still, polls indicate widespread frustration with Biden’s handling of immigration and the border, creating a political vulnerability as he seeks reelection. Homeland Security Secretary Alejandro Mayorkas told the Senate Appropriations Committee this month that the administration has been faced with a “global phenomenon” of displaced people migrating in numbers that have not been seen since World War II.

“It is unanimous that our broken immigration system is in dire need of reform,” Mayorkas said.

Democrats have other immigration priorities, such as expanding legal immigration pathways or work authorizations for migrants already in the U.S. Democrats have also warned about the danger of delaying aid to Ukraine as it enters another winter of war against Russia.

Illinois Sen. Dick Durbin, the second-ranking Democrat in the Senate, said it’s a mistake to create a situation where “we have to do significant immigration reform in the next few weeks or we won’t send money to assist the people in Ukraine or other causes important to our national security.”

Republicans have so far been adamant about the need to address Ukraine and the border at the same time.

What’s likely not on the table

Lawmakers seem unlikely to address one of the nation’s long-standing immigration issues: granting some form of permanent legal status to thousands of immigrants who were brought to the U.S. illegally as children. Republicans have made clear that will not be addressed in this package, which they want to be more narrowly focused on border security measures.

As Congress struggled to pass a comprehensive immigration overhaul, President Barack Obama launched the Deferred Action for Childhood Arrivals program in 2012 to shield those immigrants from deportation and allow them to work legally in the country. But it has been caught up in the courts ever since, and Trump, the front-runner for the Republican presidential nomination in 2024, tried to end it when he was in the White House.

Democratic Sen. Chris Murphy of Connecticut, one of the Senate negotiators, would not say early last week whether his side had proposed DACA provisions as part of the talks. But he said any deal “has to respect both Republican and Democratic priorities.”

“The more Republicans want, the more Democrats are going to want,” Murphy said.

Republicans argue that Ukraine aid could be a tough sell to some of their voters, and the border policy is the compromise.

Arkansas Sen. Tom Cotton, a Republican who has been involved in the talks, said before the Thanksgiving holiday that the negotiations were not “very close yet, because Democrats have not yet accepted that the negotiations are not border security for Democratic immigration priorities. It’s border security for Ukraine aid.”

So far, leaders in both parties have encouraged the talks. But as senators restart their work and face pressure to approve funding by the end of the year, some are warning that a narrow deal is likely the best that they can do.

“I don’t think it’s realistic to solve anywhere close to the whole problem in the next two weeks,” Murphy said.