In Colorado, the Teamsters and the United Food and Commercial Workers -- two of the state's largest unions -- are backing former state House Speaker Andrew Romanoff in the Democratic Senate primary. The SEIU, meanwhile, is backing Sen. Michael BennetMichael Farrand BennetDemocrats protest Trump's immigration policy from Senate floor On The Money — Sponsored by Prudential — Senators hammers Ross on Trump tariffs | EU levies tariffs on US goods | Senate rejects Trump plan to claw back spending Senators hammer Ross over Trump tariffs MORE (D-Colo.).

Romanoff has been able to get traction with unions because he's a known quantity, said Dean Toda, a spokesman for his campaign. During his tenure in the Statehouse, "he has gotten to know labor unions quite well and members of those unions have gotten to know him quite well."

Bennet, however, is an "unknown quantity," Toda said. "Still they don't know enough about him yet, which is why I think so many of them prefer Andrew."

Craig Hughes, Bennet's campaign manger, said: "In the last year in the Senate, Michael's built a lot of relationships, made a lot of friendships and is working very hard for Colorado's working families."

Hughes also pointed to the backing of the state branch of the American Federation of Teachers (AFT) as a show of union support. But the incumbent doesn't have a lock on education labor groups.

The major union endorsement still outstanding is the Colorado Education Association (CEA), which represents teachers and support personnel. The group is mulling its options now and state-based strategists said when they do endorse, Bennet, who clashed with teachers as superintendent of Denver Public Schools, may not be the group's first choice. Still, said Hughes, "we're hoping to get the endorsement of the teachers."

The CEA will host a delegate assembly at the end of April and issue an endorsement "right after," an official said.

Meanwhile, the Colorado AFL-CIO will endorse in June.