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 House Republicans have already rejected it.
"Any consideration of this Senate bill in its current form is a waste of time. It is DEAD on arrival in the House," the top four Republican leaders said in a joint statement on Monday.
"We encourage the U.S. Senate to reject it."
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.
In a speech on the Senate floor on Monday evening, Republican Senate Minority Leader Mitch McConnell urged his colleagues to support the measure.
"The gaping hole in our nation's southern borders on President Biden's watch is not going to heal itself," he claimed.
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.
But Republicans began rejecting the measure before any legislative text had been unveiled, alleging its border reforms did not go far enough.
What's happening at the border?
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."
What's in the new deal?
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 reach a one-week average threshold of 5,000 a day or exceed 8,500 in a single day.
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 and it is not clear whether it can cross that threshold.
"The 'border deal' is an easy NO. It reads like a parody of an actual border security bill," said Marco Rubio, a Florida Republican senator.
Widespread opposition to the deal among House Republicans means that the immigration bill is unlikely ever to become law.
Many have demanded stricter asylum restrictions, limiting programmes allowing migrants to live and work in the US while they wait for hearings.
House Republican leaders claimed that the bill cannot pass their chamber because it "fails in every policy area needed to secure our border and would actually incentivize more illegal immigration".
It has prompted Democrats to accuse their counterparts of bowing to pressure from Mr Trump, who has urged his Capitol Hill allies to kill the bill.
On Monday morning after the details of the deal emerged, Mr Trump took to his Truth Social platform and said "only a fool, or a radical left Democrat" would vote for the "horrendous" bill.
"This bill is a great gift to Democrats," he added. "And a death wish for the Republican Party."
Experts say 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. "They want more B-roll of chaos during the campaign."
But some Democrats to the left of the party on immigration have also expressed displeasure with the bill.
Pramila Jayapal, a Washington congresswoman, said its provisions are "not serious reform and it once again throws immigrants under the political bus".
"I cannot support a proposal that fails to learn from 30 years of data and would only repeat our mistakes — with migrant lives in the crosshairs," she said.