Flake, who plans to retire at the end of his term, compared Trump to Soviet dictator Josef Stalin in a speech on Wednesday attacking Trump's stance toward the press. The White House has said it plans to follow through on Trump's promise to hand out satirical "fake news" awards to reporters on Wednesday.
"I found it quite interesting that he is coming out to attack this president considering he's one that was recently defending an actually oppressive regime," Sanders said of Flake. "He went to Cuba a few weeks ago and served as a mouthpiece for the Cuban government."
Flake has long criticized Trump, and the president has in the past returned fire.