"Washington is broken and Rep. Tim Walberg is part of the problem," Byrnes says in a statement obtained by The Hill. "Michigan's middle-class is paying the price for his partisan politics that put special interests and corporations before working families."

The seat leans slightly Republican, giving Mitt Romney 51 percent of its vote in 2012, and is the type Democrats need to win to have a shot at taking back control of the House next year.

Walberg first won his seat in 2006 by defeating another Republican congressman in a campaign helped by big spending from the conservative Club for Growth. He lost his seat in the 2008 Democratic wave in a tight race with Rep. Mark Schauer (D-Mich.), then defeated Schauer during the 2010 GOP wave before winning by a double-digit margin in 2012.

Byrnes served three terms in Michigan's statehouse before losing a Democratic primary for a state senate seat in 2010. She has a centrist reputation and was one of the few Democrats to back a plan to change teacher pensions in the state, which may have cost her labor support in that race.