OLYMPIA — Mark Miloscia, who served for 12 years as a Democrat in the state House, announced Wednesday he is running for the state Senate as a Republican.

Miloscia, who left the House when he ran unsuccessfully for state auditor in 2012, is challenging fourth-term Democrat Tracey Eide in South King County’s 30th District.

His entrance should make the race one of the most competitive of the year, a boon for the GOP-led Majority Coalition Caucus that currently holds a two-seat advantage in the chamber. That majority includes two Democrats who crossed party lines to caucus with Senate Republicans.

The 30th is a swing district that is represented by one Democrat and one Republican in the House.

It is one of the few districts where Republicans are playing offense in 2014; they will have to defend several swing seats against Democrats determined to take back the Senate.

Democrats control the state House and governor’s office.

Miloscia, a 55-year-old Air Force veteran and former substitute teacher, has a reputation as a fiscal moderate and social conservative. While in the House, he chaired the housing committee as well as the Audit Review and Government Oversight Committee.

In a news release, Miloscia said he switched parties because “Washington state Republicans don’t insist that everyone believe and vote exactly the same way to run for office.”

He has already won endorsements from several prominent Republicans, including former U.S. Senate and gubernatorial candidate Dino Rossi.

Brian M. Rosenthal: 206-464-3195 or brosenthal@seattletimes.com.