Latest Dem to announce he's running: Michael Bennet

Mr. Bennet now faces the challenge of setting himself apart in the Democratic field and competing against better-known candidates, some of whom have been laying the groundwork for their campaigns for months or years.

Among them is Mr. Hickenlooper, who once hired Mr. Bennet as his chief of staff. “They are friends,” said Mr. Hart. “They will be polite to each other. They’re not going to go on background and stick a knife in one another.”

Mr. Bennet was asked on Thursday about the high number of Democrats running (he is the 21st to declare his candidacy). “This is the opportunity for us to show what we stand for, for us to have a competition of ideas,” he said. “I think it’s phenomenal that we’ve got as diverse an array of candidates as we have, in all respects, and that we’ve got the number that we have.”