"We have very, very weak laws," the President said, demanding an end to the process known as 'catch-and-release,' which a year ago Mr. Trump announced would be ended by his administration.
"We have the worst laws," the President added to more applause.
With Rep. Evan Jenkins (R-WV) and state Attorney General Patrick Morrissey (R) at his side - both running for U.S. Senate against Sen. Joe Manchin (D-WV) - the President went after Manchin several times.
"We have very weak laws because of the Democrats and Joe," the President said, criticizing Manchin and his allies for not backing his tax and immigration plans.
Admitting at one point that he was off script at his tax event, the President also revived - and expanded - his unproven claim about massive voter fraud, which in the past he said had cost him the popular vote majority in 2016.
"They always like to say, 'Oh, that's a conspiracy theory,'" the President said of his unsubstantiated charge that between 3-5 million illegal votes were cast in 2016.
"Not a conspiracy theory, folks," the President said, as he claimed that many people in California vote multiple times. "Millions and millions of people."
In the past, the President has said - without any evidence to back up the charge - that illegal immigrants had cast many votes in California.
But this time, he said his opponents were voting multiple times, "in many places."
The President created a commission to look into his charge of massive voter fraud in 2016, but it never produced a report, and gave no indication of any evidence of substantial fraud.
In a recent trial in Kansas, state Attorney General Kris Kobach - who led that federal voter fraud panel - could only produce evidence of 11 non-citizens voting since 2000 in the Sunflower State, far short of the level of massive fraud that some in the GOP have made.