"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. Policy differences don't matter right now. History has its eyes on us," Comey wrote in a Twitter post on Tuesday night.
Comey was fired in May 2017 over what the Justice Department said was his handling of the FBI investigation into 2016 Democratic presidential candidate Hillary Clinton, although Trump himself later linked the firing to the bureau's Russia probe.
The FBI began an investigation of Russian meddling in the 2016 presidential election that was taken over after Comey's firing by the Justice Department-appointed special counsel, Robert Mueller.
Comey has become a sharp critic of Trump since his firing. In testimony before Congress in June 2017, he said the new president behaved inappropriately in private meetings where he sought a pledge of personal loyalty. Trump has denied he sought a loyalty pledge from Comey.
(Reporting by Doina Chiacu; editing by Jonathan Oatis)