The Trump administration branded Chicago a "war zone" on Sunday as a justification for deploying soldiers against the will of local Democratic officials, while a judge blocked the White House from sending troops to another Democratic-run city.

An escalating political crisis across the country pits President Donald Trump's anti-crime and migration crackdown against opposition Democrats who accuse him of an authoritarian power grab.

In the newest flashpoint, Trump late on Saturday authorised the deployment of 300 National Guard soldiers to Chicago, the third-largest city in the United States, despite the opposition of elected leaders, including the mayor and state Governor JB Pritzker.

Homeland Security Secretary Kristi Noem defended the move on Sunday, claiming on Fox News that Chicago is "a war zone."

But Pritzker, speaking on CNN's "State of the Union" show, accused Republicans of aiming to sow "mayhem on the ground. They want to create the war zone, so that they can send in even more troops."

"They need to get the heck out," he said.

A CBS poll released on Sunday found that 42 percent of Americans favour deploying the National Guard to cities, compared to 58 percent who oppose it.

But Trump's campaign to use the military on home soil hit a roadblock late on Saturday in Portland, Oregon, when a court ruled the deployment was unlawful.

Trump has repeatedly called Portland "war-ravaged," but US District Judge Karin Immergut issued a temporary block, saying "the president's determination was simply untethered to the facts." (AFP)