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Republican Senator Jeff Flake has told The Washington Post that he would support impeachment proceedings against Donald Trump if the president ends Special Counsel Robert Mueller's investigation into Russian interference in the 2016 presidential election "without cause".

“If [Trump] fires [Mueller] without cause, how different is that from what Nixon did with the ‘Saturday Night Massacre’? He left before impeachment came, but that was the remedy then and that would be the remedy now," Flake said.

Flake has been a prominent critic of the president, but his warning came days after similar salvos from Republican elder Senator Orrin Hatch, who said firing Mueller would be a "foolish action" and "the stupidest thing" the president could do.

Trump has stepped up his criticism of Mueller, attacking him by name for the first time on Twitter in the days following the president's decision to fire FBI deputy director Andrew McCabe – 26 hours before his scheduled retirement.