"And I think he’s got the best chance to hold the seat in November," Clinton added.
He never appeared in person for Romanoff.
Obama, meanwhile, was in Colorado in February to fundraise for Bennet, and he called into the senator's "tele-town hall" last week.
"Michael has been as good of a senator as I expected him to be," the president said during the event. He added: "I know there have been a lot of negative ads running against Michael in the last few weeks, which is sort of politics as usual. But when he came to Washington, he came to get things done and not just play the usual political games."
Clinton has been a successful surrogate for Democrats in tough contests. In May, he headlined an event for Rep. Mark Critz (D-Pa.) in the final days of Pennsylvania's special election to fill the late Rep. John Murtha’s (D-Pa.) congressional seat. Critz pulled off a somewhat unexpected win in the Republican-leaning district.
In early June, Clinton went to Arkansas to campaign for embattled Democratic Sen. Blanche Lincoln, who ended up winning her primary runoff.
Meanwhile, Obama has had a mixed record of endorsing candidates. Earlier this cycle he backed Sen. Arlen Specter (D-Pa.) to win his primary over Rep. Joe Sestak (D-Pa.). Sestak ended up winning that contest.