Donald Trump has vowed to send federal police officers into several more Democrat-run American cities amid ongoing Black Lives Matter protests.
The US president is under fire for defying constitutional norms and attempting to play the “law and order” card to boost his re-election campaign this autumn.
It follows violent scuffles between federal officers and protesters in Portland, Oregon, involving agents in camouflage, tear gas fired and activists being bundled into unmarked cars.
Critics have accused the White House of stoking - rather than quelling - racial tensions that remain simmering across much of the US following the death of George Floyd, an unarmed African American, in Minneapolis police custody in May.
"We're going to have more federal law enforcement, that I can tell you," Mr Trump said on Monday. "In Portland, they've done a fantastic job. They've been there three days, and they really have done a fantastic job in a very short period of time."
The Chicago Tribune, citing anonymous sources, reported Monday that Trump planned to deploy 150 federal agents to Chicago.
But constitutional law experts and politicians said federal officers' actions in the progressive city of Portland are a "red flag" in what could become a test case of states' rights as the Trump administration expands federal policing.
"It is a standard move of authoritarians to use the pretext of quelling violence to bring in force, thereby prompting a violent response and then bootstrapping the initial use of force in the first place," said Michael Dorf, a professor of constitutional law at Cornell University.
Oregon Governor Kate Brown, a Democrat, said on Twitter: "This is a democracy, not a dictatorship. We cannot have secret police abducting people in unmarked vehicles. I can't believe I have to say that to the President of the United States."
Republican Senator Rand Paul of Kentucky added: "We cannot give up liberty for security. Local law enforcement can and should be handling these situations in our cities but there is no place for federal troops or unidentified federal agents rounding people up at will.”
The protests have rocked Portland for 52 nights, with around 500 gathering outside the federal courthouse overnight on Sunday.
Many rallies have attracted thousands and been largely peaceful but have boiled over into violence at times setting fire to police precincts, smashing windows and clashing violently with local police.
