Speaking at a campaign rally in Greensboro, North Carolina, Trump appeared to be suggesting, as he has in the past, that Democrats are hoping to convert migrants who enter the country illegally into reliable voters.

Biden's administration, Trump contended, seeks "to collapse the American system, nullify the will of the actual American voters and establish a new base of power that gives them control for generations." He did not elaborate.

In past statements, Trump has suggested that Democrats are purposefully allowing migrants into the country to grow their political support, a longstanding claim espoused on the far-right known as the "great replacement theory."

The Biden campaign did not immediately reply to a request for comment.

Only U.S. citizens can vote in federal elections. But Trump has made border security a central tenet of his campaign as polls show voters in both parties becoming increasingly concerned about the steady stream of migration.

Both Biden and Trump toured the southern border along Texas in separate visits on Thursday, a sign they each view the issue as politically potent.

Trump frequently claims without evidence, as he did again on Saturday, that migrants have caused a spike in violent crime in U.S. cities.

At the North Carolina rally, Trump called the influx of migrants an "invasion" and said Biden would "turn our public schools into migrant camps."

"We are not going to let them turn the USA into a crime-filled, disease-ridden dumping ground," Trump said.

Under pressure from Republicans who accuse him of failing to control the border, Biden called on Congress last year to provide more enforcement funding and said he would "shut down the border" if given new authority to turn back migrants.

Last month, however, a bipartisan immigration bill stalled in the U.S. Senate after Trump told Republicans not to support it even though it contained several border-security measures they had sought.

Jennifer Mercieca, a professor at Texas A&M University who has written a book about Trump's rhetoric, said he often uses unfounded conspiracy allegations to undermine opponents with "self-sealing narratives" that can't be proven true or false.

"Previously he's said that Biden's weakness was allowing weakness at the border, but here it's a plot," she said. "Trump has prevented the border bill from passing so that he can make these accusations against Biden."

Trump was campaigning in North Carolina ahead of its primary on Tuesday, one of 16 nominating contests that will be held across the country that likely will push him close to clinching the Republican presidential nomination.

(Reporting by James Oliphant in Greensboro, North Carolina; Additional reporting by Nathan Layne; Editing by Colleen Jenkins and Jonathan Oatis)

By James Oliphant