Comey tweeted on Tuesday that he voted in his first Democratic primary and he believes the country needs a candidate "who cares about all Americans and will restore decency, dignity to the office".

Comey says there's a reason Trump "fears" Biden and "roots" for Vermont senator Bernie Sanders.

Trump frequently targets Biden on Twitter, calling him "Sleepy Joe Biden" and recently mocking his debate performance.

The president also tweets about Sanders, saying the Democrats are "staging a coup against Bernie!"

Comey has served in both Republican and Democratic administrations. He was fired as FBI director by Trump in May 2017 and has been a chief antagonist of the president's since then.

Fourteen states are voting on Super Tuesday in the Democratic battle to find a challenger to Republican President Trump at the November election.

Australian Associated Press