WASHINGTON — Senators negotiating new U.S. immigration restrictions as part of a deal to advance Ukraine aid are exploring ways to prevent a future president from abusing some of the executive powers on the table to control the number of migrants seeking asylum, two sources with knowledge of the talks said.

One source said Democrats want a “safety valve” in the new policies to avoid cruelty and mass roundups that could someday be applied far beyond the goal of controlling the border. They’re conscious that enhanced powers without limits may backfire in case former President Donald Trump — or someone with similar attitudes on immigration — gets elected.

It’s the subject of some key sticking points as Republicans demand more expansive powers for the president to crack down on a border situation they describe as uncontrolled and chaotic.

In one dispute, GOP senators want to expand a president’s discretionary powers to close the border. While Democrats aren’t worried about President Joe Biden abusing that power, “what Donald Trump could do with that — and would do with that — is very different. And that is clearly on people’s minds as they are drafting,” one Democratic source familiar with the talks said.

“None of this is being drafted in a vacuum,” the source said.

Another central example is that Democrats are open to beefing up expedited removal powers to allow an administration to turn away new migrants — but they’re adamantly opposed to expanding that to deportations from inside the country, which Trump tried and failed to do as president. “That would never fly amongst Democrats,” the Democratic source told NBC News, calling it a “red line” for them.

Democrats worry that beefed-up expedited removal authorities without clear limitations — particularly under a new Republican administration could lead to mass targeting and racial profiling, causing the deportation of longtime residents, spouses of Americans and maybe even U.S. citizens themselves.

Some progressives say Democrats should reject new policies to crack down on the border entirely, citing Trump’s remarks accusing immigrants of “poisoning the blood of our country” — which critics have compared to comments by Adolf Hitler — and his statement to the New York Young Republican Club that he wants “to be a dictator for one day.”

Sen. Alex Padilla, D-Calif., said that Trump’s recent “pledges to serve as a dictator and his vile anti-immigrant rhetoric should be a wakeup call” not to give a president more powers “to target, round up and deport immigrants.”

“Expanded nationwide expedited removal is an incredibly dangerous tool that, if willingly handed over in these negotiations, could be easily abused by a future Trump administration to target political opponents and critics,” Padilla said.

But the White House and Democratic leaders have a different answer: Cut a deal but circumscribe the new powers to prevent abuse by Trump or another future president.

“As the Article I branch, we should always have an interest in not outsourcing too much authority to the Article II branch, as a matter of principle,” Sen. Chris Murphy, D-Conn., the party’s lead negotiator on the issue, told NBC News.

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Republicans, meanwhile, want more aggressive authorities and mandates for the executive branch regardless of the next election, fearing that Biden will be too lenient with asylum seekers and fail to reduce flows at the border.

Sen. James Lankford, R-Okla., the top Republican negotiator, said the Biden White House has made clear they want more tools to manage the border, arguing that it’s not purely a GOP position.

“They’re saying ‘We’d like to be able to solve some of these things, but we don’t have the authorities to be able to do it.’ We need those authorities,” Lankford said.

That has been the fundamental tension at the heart of the elusive deal between Democrats and Republicans, who said they continued to negotiate and make progress over the weekend, continuing into Monday without an agreement. Biden is motivated to reach an immigration deal as the GOP has made clear it is a condition to win their votes to advance aid to Ukraine and Israel.

As a compromise, the two sides are exploring a “trigger” that would initiate expedited removal at the border — for instance, a certain number of migrants seeking asylum or apprehended between ports of entry, said Sen. Thom Tillis, R-N.C. He theorized that the threshold could be 3,000 asylum-seekers per day, taking discretion away from the president and requiring the president to use those authorities to begin turning away migrants.

“Now, the key is the language would have to be so explicit that we could literally win a lawsuit quickly, that the president is not exercising his authority,” Tillis said, of the GOP position. “So that gets into language: When it’s time to shut the border because we can’t handle the capacity, the president must act.”

The text of any agreement will be key to evaluating whether it has the votes to pass the Senate, where Democrats control 51 votes and need 60 to break a filibuster, as well as the Republican-led House, where Speaker Mike Johnson has demanded more far-reaching policies to restrict migration. And Biden will face his own pressures from immigration advocates in his party as he assails Trump’s anti-immigrant language.

Sen. Bob Menendez, D-N.J., said last week that “the administration’s proposals read like a bucket list of Stephen Miller’s wildest fantasies,” referring to the immigration hardliner who advises Trump and played a key role in shaping his policies as president.

Todd Schulte, the president of the bipartisan pro-immigration group FWD.us, noted that Trump’s administration sought to expand the powers being discussed, to institute an immigration crackdown.

“There is no need to speculate on what President Trump and his team want for their far-right, authoritarian agenda,” he said. “They have explicitly stated that nationwide expedited removal is absolutely fundamental if they’re going to pursue their horrific efforts to arrest and deport millions and millions of immigrants.”

Tillis said he’s not sympathetic to the pushback from the left as talks continue, instead saying it may even help win GOP votes for a deal.

“A part of it may be some apprehension that people are going to get shots — that their base is somehow gonna come after them. Cry me a river,” said Tillis, noting that he has been censured by his state party for backing bipartisan measures. “Bipartisan stuff is hard. They need to step up and do it.”