"Republicans used to understand that the actions of a president matter, that the words of a president matter, that the rule of law matters and that the truth matters," Comey continued, urging Republicans to "stand up for the values of this country" and calling them "shameful."

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Trump has repeatedly attacked the FBI's role in investigating possible coordination between Russia and his campaign in 2016.

Comey has been singled out for particular scorn. Earlier this year, Trump called him an "untruthful slimeball" in response to the publication of a book by Comey that characterised Trump as an ego-driven congenital liar.

After Comey spoke to reporters on Tuesday night, Sanders wrote on Twitter that "Republicans should stand up to Comey and his tremendous corruption," adding that Trump "did the country a service by firing him and exposing him for the shameless fraud he is."