Donald Trump is "morally unfit" to lead the U.S., former FBI Director James Comey said in a highly anticipated prime-time interview Sunday, adding that he can't rule out the possibility the Russian government has incriminating information about the president.

"I think it's possible. I don't know," Comey said in an interview with ABC News' Chief Anchor George Stephanopoulos, when asked whether Trump has been "compromised" by the Russians. "These are more words I never thought I'd utter about a president of the United States, but it's possible."

Comey said he had concluded during the presidential campaign that Trump was dishonest. Americans should be united, he said, in the idea that "our president must embody respect and adhere to the values that are at the core of this country. The most important being truth. This president is not able to do that. He is morally unfit to be president."