Ten months ago, Michigan Democrats took full control of state government for the first time in 40 years.

Despite slim legislative majorities, they wielded that newfound power quickly and decisively, enacting gun laws, codifying civil rights for L.G.B.T.Q. people, solidifying abortion rights and undoing Republican laws that they said weakened labor unions.

But on Tuesday night, Democrats learned that their 56-to-54 advantage in the Michigan House of Representatives would become a 54-to-54 tie, at least temporarily, because two Democratic legislators had won elections to become mayors of Detroit suburbs.

When the two vacate their House seats, as they are required to do in the coming days, Michigan’s leftward march of 2023 is likely to reach a partisan stalemate.

Michigan was one of four states with a new Democratic trifecta — single-party control of the governorship and both legislative chambers — when this year began. Bolstered in last year’s election by redrawn legislative maps, an abortion ballot question and a strong showing by Gov. Gretchen Whitmer, who decisively won a second term, Michigan Democrats narrowly flipped control of both the State House and Senate.

Ms. Whitmer, a Democrat who is frequently mentioned as a possible future presidential candidate, is expected to call special elections to fill the seats of the soon-to-be mayors, Representatives Kevin Coleman and Lori M. Stone. Mr. Coleman is leaving the Legislature to become mayor of Westland, population 84,000, and Ms. Stone will become mayor of Warren, population 137,000. Both are expected to remain in the Capitol for the next few days.

Mr. Coleman and Ms. Stone each won legislative races in their districts last year by more than 25 percentage points, leaving Democratic leaders confident that the party will ultimately reassert its trifecta.

But special elections can be unpredictable. And even if Democrats hold both seats, many expect it will be a few months before new legislators take their oaths of office in Lansing. By that point, the state will be in the throes of the 2024 election cycle, in which Republicans hope to win back the House.

In recent months, some Democrats had worried that the two mayoral races could possibly disrupt their party’s narrow hold on Michigan, a perennial battleground that Donald J. Trump won in 2016 and Joseph R. Biden Jr. won in 2020.

Mr. Coleman said he was proud of the Democrats’ record this year in Lansing, especially on labor issues, and was excited about the chance to lead his working-class suburb. His new job carries a pay raise and spares him an 80-minute commute to Lansing. But the win, he said, was in some ways bittersweet.

Mr. Coleman said some fellow Democrats, including members of Ms. Whitmer’s staff and Speaker of the House Joe Tate, expressed concerns to him about the mayoral run. But none of them, he said, did anything to undermine his campaign for mayor.

“My perception is she trusted my decision to run, and I think she understood where I was coming from,” Mr. Coleman said of the governor. “I think she understands, again, our seat’s not going anywhere, it’s just a matter of maybe it slows the process down a little bit.”

Ms. Stone did not immediately respond to an interview request.

Ms. Whitmer’s office did not make her available for an interview or immediately respond to Mr. Coleman’s characterization of her views. A spokeswoman for the governor, Stacey LaRouche, said in a statement that “this legislative term has been one of the most productive in state history” and that Ms. Whitmer had accomplished many of her top priorities.

Democrats said they believe Mr. Tate will remain the speaker and Democrats will retain committee chairmanships while the House is evenly divided, and Republican leaders have not indicated that they will challenge that. But the chamber is unlikely to pass bills without bipartisan support until the vacant seats are filled.

Representative Matt Hall, the Republican leader in the House, congratulated the “two fine public servants” on their mayoral wins in a statement and said “we are entering a new era in Lansing with a House that is now evenly divided.”

He added: “House Democrats have a choice to make: Together we can forge compromise and achieve the most productive months of the session, or the House Democrat leadership can take their ball and go home until next spring.”

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