Former FBI Director James Comey called on Americans to vote for Democrats in the wake of President Donald Trump's summit with Russian President Vladimir Putin, slamming Republicans for not pushing back against the president.

"This Republican Congress has proven incapable of fulfilling the Founders' design that 'Ambition must ... counteract ambition.' All who believe in this country's values must vote for Democrats this fall," he wrote in a Tuesday night tweet.

Comey, who was fired by Trump for his role in the Russian meddling investigation, pointedly condemned the president's summit and news conference with Putin on Monday, saying Trump had "sold out our nation on an international stage" and calling for "patriots" to "reject the behavior of this president."

"Policy differences don't matter right now,” Comey wrote in the tweet. "History has its eyes on us."

The president came under widespread, bipartisan condemnation on Monday and Tuesday for not backing his intelligence community's assessment that Moscow had interfered in the election. But GOP leaders in Congress have taken no action against Trump.

Comey was the head of the FBI until May of last year, when the president fired him. Trump initially said he did so at the urging of the Justice Department because of Comey's handling of the Hillary Clinton email investigation during the 2016 election; days later, he told NBC News' Lester Holt that he was going to fire Comey anyway, and he was thinking about "this Russia thing" when he fired Comey.

Trump has repeatedly trashed Comey, who released a tell-all book earlier this year, calling him "slippery" and a liar in tweets.

Comey was a registered Republican for most of his life. Earlier this year, he said he felt the GOP had "left" him and "many others."

"The Republican Party, as near as I can tell, reflects now entirely Donald Trump's values," Comey said on an ABC News podcast. "It doesn't reflect values at all. It's transactional, it’s ego-driven, it’s in service to his ego."