After the Department of Homeland Security deployed scores of Border Patrol police and federal marshals — many on combat fatigues — to Portland, Oregon, last week to tackle anti-racism protests, Trump on Monday said he could do so in other Democrat-led cities.
He called the move necessary but critics labeled it an election year political stunt.
"This president blusters and bluffs and says he's gonna do things and they never materialize on a regular basis," de Blasio told reporters.
"If he tried to do it, it would only create more problems, it would backfire, wouldn't make us safer and we would immediately take action in court to stop it. From my point of view, this would be yet another example of illegal and unconstitutional actions by the president," the mayor added.
Trump's decision to have military-clad federal law enforcement authorities intervene in local protests has sparked anger and legal questions.
Since African-American man George Floyd was killed by a Minneapolis police officer on May 25, sparking nationwide protests against racism and police brutality, Trump has sought to paint the demonstrators as radical leftists intent on destroying the country.
Facing an uphill battle for reelection in November against Democrat Joe Biden, Trump is using the protests to rally support from his conservative base, Democrats say.
"We're looking at Chicago, too. We're looking at New York," Trump told reporters on Monday, also naming Philadelphia, Detroit and Baltimore.
"Look at what's going on. All run by Democrats, all run by very liberal Democrats, all run really by radical left. We can't let this happen to the city."
According to reports, the DHS was preparing to send 150 paramilitary personnel to Chicago this week after police there clashed with demonstrators seeking to tear down a statue of Christopher Columbus.