President Joe Biden is expected to announce Tuesday that he will temporarily shut the US-Mexican border to migrants whenever illegal crossings surge, in a dramatic bid to neutralize one of his political weak spots in the reelection battle against Donald Trump.

The 81-year-old Democrat is set to sign a long-awaited executive order that would allow officials to bar migrants who enter illegally from receiving asylum when numbers top 2,500 in a day -- and then make it easier to deport them back to Mexico.

Biden will deliver remarks at 2pm (1800 GMT), the White House said, without giving further details.

The tough new curbs could take effect immediately as the numbers of people crossing the border without proper documentation are already above the threshold, US media reported. Asylum seekers would again be allowed to enter once the numbers dropped to 1,500 a day.

The move would be one of the toughest ever by a Democratic president and see Biden moving further towards Republican Trump's own signature border policies, amid polls showing the issue heavily dragging on Biden's reelection chances in November.

Trump's campaign dismissed the expected executive order as being "for amnesty, not border security."

The statement repeated Trump's frequent claims that illegal migrants are responsible for surges in violent crime -- an allegation not supported by any major police or academic data.

The White House would not confirm reports that Biden will sign the executive order at a ceremony alongside mayors representing border towns.

"What I can say is we are constantly and continuously looking at all options to try and really deal with the immigration system, a system that's been broken for decades," Press Secretary Karine Jean-Pierre said on Monday.

Jean-Pierre blamed Republicans in Congress for failing to cooperate with Biden, and for blocking billions of dollars in funding for the border which the president tried to push through along with money for Ukraine and Israel.

"They decided to pick partisan politics," she said.

 Trump rhetoric 

Biden aims to defuse Republican attacks and to woo middle-of-the-road voters worried about the state of the border.

But his plan will anger leftist Democrats and it will almost certainly be challenged in court by civil rights groups.

More than 2.4 million migrants crossed the southern US border in 2023 alone, largely from Central America and Venezuela as they flee poverty, violence and disasters exacerbated by climate change.

Growing numbers of migrants from other parts of the world are also making the journey to Latin America and then the treacherous trip north into the United States.

The figure rose to a record high of 10,000 a day in December and, while it has fallen dramatically in recent months, polls show the issue is one of Biden's biggest liabilities in the election.

Trump spent his time in office trying to build a wall on the Mexican border and has drastically ramped up his anti-immigration rhetoric as he seeks a White House comeback.

He has repeatedly spoken of migrants "poisoning the blood" of the United States and says he would enact the biggest deportation of migrants in history as soon as he takes office.

Biden's administration has tried to curb crossings by working with Mexico and other countries to reduce migrant flows through enforcement and economic policies, but polls show voters do not think he is doing enough.

The US president's announcement was due a day after he spoke with Mexico's first female president-elect, Claudia Sheinbaum, to offer his congratulations and pledge a "strong and collaborative partnership."