US senators have unveiled a long-awaited cross-party deal that aims to combat illegal immigration at the US-Mexico border - and, in return, grant new aid to Ukraine and Israel.

The bill would introduce much more severe measures to try to stem border crossings, which are at record levels.

The influx is one of the biggest political headaches facing Joe Biden.

The Democratic-led Senate will vote on the bill this week, but a senior House Republican said the bill was doomed.

"Let me be clear: The Senate Border Bill will NOT receive a vote in the House," said Representative Steve Scalise, the majority leader in the House of Representatives, where many Republicans want the bill to be tougher.

Included in the huge $120bn funding deal is $60bn to support Ukraine in its fight against Russia and $14bn in security assistance for Israel.

The military aid became part of a Mexico border deal because Republicans had said they would not agree to more money being sent to Ukraine until action was taken to fix the migrant crisis.

Immigration has emerged as the top issue driving Republicans to the polls in support of former President Donald Trump, the front-runner to face Mr Biden in the November general election.

Faced with mounting public anger over the migrant inflows, President Biden vowed in January to "shut down the border right now and fix it quickly" if Congress sent a bill to his desk.

The bill, which senators of both parties have spent months negotiating, was unveiled on Sunday night.

Since President Biden took office in January 2021, more than 6.3 million migrants have been detained crossing into the US illegally between points of entry, according to statistics from the Department of Homeland Security, or DHS.

Of these, about 2.4 million were allowed into the US, where the majority wait for immigration court dates in which they can make a case for asylum. The system is so overwhelmed that this can take years.

A January poll conducted by CBS - the BBC's US partner - shows that nearly half of Americans view the situation at the border as a crisis, with 63% saying that the administration should adopt "tougher" policies.

More than two-thirds of Americans said they disapproved of Mr Biden's handling of the issue.

"Immigration is [Biden's] Achilles' heel. He is right up against the ropes on this," said Tony Payan, the director of the Center for the United States and Mexico at Rice University's Baker Institute in Texas.

"The Republicans have been very successful at maintaining the issue on the headlines, and tying Biden to what they term 'chaos' on the border and an 'invasion' of migrants."

The 370-page agreement will, in the words of Republican negotiator James Lankford, move from the current system of "catch and release" to one where migrants are detained and deported.

Senator Lankford brokered the deal with Democratic colleague Chris Murphy and independent colleague Kyrsten Sinema.

If passed into law, it would be the biggest immigration overhaul since the Reagan era in the 1980s.

Among the most significant changes in the deal is a new federal authority that mandates a complete shutdown of the border when migrant crossings pass a threshold of 5,000 in a week.

In practice, this would mean that migrants who arrive in the US illegally would no longer be allowed to request asylum and would be deported shortly thereafter.

Adam Isacson, a migration and border expert from the Washington Office on Latin America, told the BBC that the change would mark a "radical" departure from current norms.

The new bill, he says, reverts to the spirit of the Trump presidency which took a notably hard line on immigration, introducing Title 42, a pandemic-era policy that allowed for the rapid expulsion of migrants.

Other reforms included in the deal are fast-tracked decisions on asylum cases, limits on humanitarian parole, expanded authority to remove migrants from the US, stricter consequences for illegal crossings and even $650m to build or reinforce miles of border wall.

Collectively, Mr Isacson said these measures would have, not long ago, been largely considered unthinkable in US politics.

Before Donald Trump, these kinds of measures were not in the mainstream debate, he said.

"It was something that maybe people on the anti-immigrant fringe proposed. It really shows how much the window has shifted."

In endorsing the deal on Sunday, Mr Biden called it "the toughest and fairest" border reforms in decades.

"It would give me, as president, a new emergency authority to shut down the border when it becomes overwhelmed. Get it to my desk so I can sign it into law immediately."

The bill needs at least 60 votes to advance through the 100-member Senate.

But widespread opposition to the deal among House Republicans means that the immigration bill is unlikely ever to become law. Some Democrats on the left of the party may also be unhappy.

Even before its details were announced and his colleague Mr Scalise spoke out against it, House Speaker Mike Johnson warned that the deal would be "dead on arrival" in the chamber.

Some Republicans have demanded stricter asylum restrictions, limiting programmes allowing migrants to live and work in the US while they wait for hearings.

This Republican opposition has prompted Democrats to accuse Mr Johnson and others of bowing to pressure from Mr Trump, who has urged his Capitol Hill allies to kill the bill.

"Call it the 'stupid bill' and make sure it doesn't get passed," the ex-president wrote on his Truth Social platform on Wednesday, claiming the deal "will make things MUCH WORSE".

Experts say that Mr Trump's influence has cast a shadow over the negotiations.

"Letting the Biden administration twist in the wind is exactly what the Trump campaign wants," said Mr Isacson.

"That sort of status quo, of They want more B-roll of chaos during the campaign."