The move comes as no surprise — Peters has cleared the Democratic field — but shows that the power players in the Michigan Democratic Party are united in their support of his campaign.

"It's of course important to me that the person who is elected to represent Michigan in the Senate next year be a fighter for Michigan's families and communities," Levin writes in an email to his supporters sent Thursday. "That's why I am pleased to announce today that I am endorsing Gary Peters for the U.S. Senate. I am eagerly looking forward to supporting Gary and helping him in his campaign."

While Peters has the field to himself, Republicans have yet to find a candidate for the seat. Rep. Mike Rogers (R-Mich.), who is viewed by most establishment Republicans as the party's best shot at the seat, could also opt to stay in the House, and has been discussed as a potential new head of the FBI.

Former Michigan Secretary of State Terri Lynn Land (R) is also looking at a bid but isn't as well-known. Rep. Justin AmashJustin AmashLawmakers seek to limit US involvement in Yemen's civil war NC Republican pressed on Trump in primary showdown Harassment rules play into race for Speaker MORE (R-Mich.) has been increasingly vocal about a possible run in recent weeks, but a number of establishment Republicans have expressed concerns about his libertarian-leaning views.

Even if Rogers does run, Peters may start with an edge in the slightly Democratic-leaning state: According to a recent poll from EPIC-MRA, Peters leads Rogers by 7 points in a hypothetical head-to-head matchup.