Washington DC - Donald Trump on Saturday underlined his opposition to a bipartisan immigration plan that President Joe Biden has promised to use to "shut down" the border with Mexico if it becomes overwhelmed.

With immigration seen as one of the hottest electoral issues in what increasingly looks like a Trump-Biden rematch for the White House this year, the fate of the bill being negotiated by the Senate has become a high-stakes battleground.

Biden threw his weight behind the proposed bill on Friday, insisting it would usher in the "toughest" ever set of border reforms.

"It would give me, as president, a new emergency authority to shut down the border when it becomes overwhelmed. And if given that authority, I would use it the day I sign the bill into law," he said in a statement.

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Trump has put immigration front and center of his effort to retake the White House, issuing dire warnings about the porous nature of the border while pushing back hard against Republicans supporting the Senate plan.

"A bad border deal is far worse than no border deal," the former president wrote on his Truth Social platform on Saturday, saying the current situation was a "catastrophe waiting to happen."

Following Trump's extensive lobbying, Mike Johnson, the speaker of the Republican-controlled House of Representatives, said in an open letter Friday that any such bill adopted by the Senate would be "dead in the water" and never get passed by the House.

The deal being negotiated carries high stakes, and not just for the presidential candidates.

In addition to addressing Americans' concerns about migrants arriving via Mexico, it would provide vital military assistance for Ukraine in its war against Russian invaders.

A bipartisan deal tying aid for Kyiv, a Biden priority, with money for border militarization, as Republicans have demanded, had seemed in the cards just days ago.

White House Deputy Press Secretary Olivia Dalton said Thursday that Biden's administration had been working in "good faith" with Republicans to reach a deal and hoped they would "remain at the table so we can do that."

Border residents have long sounded the alarm over human rights abuses that come with growing militarization of their communities.