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, while at the same time criticizing the Democrat for "open borders."

With immigration 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, tacking right on the issue, 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.

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

"The border is in play (politically) like it's never been in play before," he said at a campaign speech in Las Vegas on Saturday.

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.

"They're blaming it on me, I said, 'That's OK, blame it on me, please,'" Trump said Saturday. "I'd rather have no bill than a bad bill."

As Republican Texas Governor Greg Abbott and the federal government engage in a standoff over control over the border, Trump said he would give the state his "full support" and "deploy all necessary military and law enforcement resources to seal up the final section of border."

Following up on his often dark rhetoric on immigration, the former president said there was a "100 percent chance that there will be a major terrorist attack in the United States" carried out by people crossing the border.

"We will begin the largest domestic deportation operation in America," he said, as the oft-repeated campaign pledge brought cheers from the audience.

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

In addition to addressing Americans' concerns about the huge influx of 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 security, 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." (AFP-Yonhap)