More: In a blistering ruling, U.S. District Judge Patrick Schiltz said there was "no doubt" that the subpoenas were issued to damage Walz " part of what he said was a pattern of Trump administration efforts to use criminal process to punish the president's adversaries.

"Initiating a criminal investigation in order to harass political opponents or to coerce them into taking official action " particularly official action that the federal government cannot directly require those political opponents to take " is a blatantly unlawful and unethical use the grand-jury process," Schiltz wrote in a 29-page ruling dated June 17 but unsealed Monday.

The George W. Bush-appointed chief judge said Trump's repeated attacks and promises of "retribution" against Walz, a Democrat, and other Minnesota officials "establishes beyond reasonable dispute" that the grand jury subpoenas " issued at the height of ICE's Operation Metro Surge " "were a part of a broader campaign to coerce state and local officials in Minnesota to assist the Trump administration in its enforcement of immigration laws."

The federal government is barred by the Constitution from forcing states to enforce federal laws, Schlitz added.

Moreover, Schiltz linked the subpoenas to what he called "the Trump administration's well-established history of using criminal investigations to retaliate against and pressure the President's political and personal adversaries." Schiltz pointed to the recent investigation of Federal Reserve Chair Jay Powell, in which another federal judge similarly quashed subpoenas on the grounds that they were retaliatory.

The Justice Department did not immediately respond to a request for comment. Walz celebrated the decision as "a victory for the rule of law and our democracy."

The ruling rejects six grand jury subpoenas that federal prosecutors targeted at Walz, the mayors of Minneapolis and St. Paul, the Minnesota attorney general and commissioners of Hennepin and Ramsey counties.