Donald Trump visited the US-Mexico border Wednesday, pushing anti-immigrant rhetoric and rallying his base by warning that "millions" of undocumented migrants were surging into the country due to the lax policies of his presidential successor Joe Biden.

The Republican former leader's first fact-finding tour since leaving the White House comes as the Biden administration grapples with a migrant surge that Trump blames squarely on an easing of his "tough but fair" policies that were aimed at deterring new arrivals.

"Now we have an open, really dangerous border, more dangerous than it's ever been in the history of our country -- and we better go back fast," the brash billionaire said after receiving a briefing from sheriffs at the Department of Public Safety in the small town of Weslaco, Texas.

"Millions of people are coming in," Trump added later in remarks delivered along the border in Pharr, Texas, part of his recent ramp-up of public appearances.

US Vice President Kamala Harris visited a Customs and Border Protection Central Processing Center in El Paso, Texas when she visited the border region on June 25, 2021Vice President Kamala Harris is traveling in El Paso, Texas on Friday, where she will
US Vice President Kamala Harris visited a Customs and Border Protection Central Processing Center in El Paso, Texas when she visited the border region on June 25, 2021Vice President Kamala Harris is traveling in El Paso, Texas on Friday, where she will tour a Customs and Border Protection processing facility, meeting with advocates and NGOs. AFP / Patrick T. FALLON

Migrant detentions reached their highest level in 15 years in March, and Biden critics accused the president of downplaying the situation.

But the numbers are in the hundreds of thousands, not millions, and many migrants are being returned to Mexico.

Trump was accompanied by Republican Texas Governor Greg Abbott, who has pledged to finish Trump's wall along his state's border -- but with private donations and not federal assistance.

Trump was also joined by several Republican members of Congress, including conservatives Lauren Boebert and Jim Banks, as they observed an uncompleted portion of the re-enforced metal wall looming over a grassy bluff.

Former US president Donald Trump was accompanied by Texas Governor Greg Abbott (C) for a visit to a section of the border wall near Pharr, Texas
Former US president Donald Trump was accompanied by Texas Governor Greg Abbott (C) for a visit to a section of the border wall near Pharr, Texas AFP / Sergio FLORES

The area is in the Rio Grande Valley, one of the illegal migration hot spots along the 1,930-mile (3,100-kilometer) border.

Trump expressed pride in his efforts to build more than 400 miles of border barrier and to tamp down the number of migrants crossing into Texas, New Mexico, Arizona and California.

"We did a hell of a job," Trump said.

Most of Trump's new construction involved bolstering or replacing existing wall or fencing, with fewer than 50 miles of it being constructed where none stood previously.

Republican lawmakers have slammed Biden for reversing Trump programs, including his "remain in Mexico" policy, which had forced thousands of asylum seekers from Central America to stay south of the US border until their claims were processed.

Former US president Donald Trump's visit to the country's southern border with Mexico -- and a portion of his border wall -- provided an opportunity for the former president to knock his successor Joe Biden over illegal immigration
Former US president Donald Trump's visit to the country's southern border with Mexico -- and a portion of his border wall -- provided an opportunity for the former president to knock his successor Joe Biden over illegal immigration AFP / Sergio FLORES

"Things have changed so quickly and so dramatically under the Biden administration, it's been amazing and disastrous," Governor Abbott said, using fear-mongering language to portray an area where Americans are being "threatened every single day" by migrants crossing the border.

Critics have also warned of the surge in drug trafficking into the United States, particularly of fentanyl, a powerful synthetic opioid.

Biden last week dispatched Vice President Kamala Harris -- whom he has tasked with overseeing efforts to stem migration -- to the border region where she highlighted the administration's commitment to "orderly and humane" immigration policies.

Harris visited El Paso, but received withering criticism because the border city is hundreds of miles from the epicenter of the migration surge.

Trump, who may yet seek re-election in 2024 but has not announced his plans, has repeatedly knocked Biden for the border crisis.

The Democratic Party has pushed back, saying Republicans were bringing their "clown show" to the border to mislead voters.

Trump, who has been booted from social media and was impeached for inciting the deadly January 6 riot at the US Capitol, is increasingly seeking the public spotlight after months of laying low.

Last week in Ohio, at his first campaign-style rally since leaving the White House, he repeated the lie that he won November's election and that Biden prevailed only through fraud.

Perhaps eager to rejuvenate conservative voters, who polls show still largely support Trump, he returned to the theme in Texas.

"Biden is destroying our country, and it all started with a fake election," Trump said.

"If you don't have good elections, and if you don't have a strong border, you don't have a country."