Hundreds of Texas National Guard soldiers have gathered at an army facility outside Chicago as Donald Trump's threat to invoke an anti-insurrection law and deploy troops to more US cities intensifies the battle over the limits to his authority.

The Republican president on Tuesday again left open the possibility that he might use the centuries-old Insurrection Act to sidestep any court rulings blocking the dispatch of Guard troops into Democratic-led cities, over the objections of local and state officials.

A federal judge has temporarily barred Guard troops from heading to Portland, Oregon, though a separate judge has allowed for now a deployment to proceed in Chicago, where federal agents have embarked on a sweeping crackdown on illegal immigration.

"Well, it's been invoked before," Tr

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