Rep. Jim MathesonJim MathesonTrump's budget targets affordable, reliable power Work begins on T infrastructure plan New president, new Congress, new opportunity MORE (D-Utah) won't run for reelection, he announced Tuesday afternoon, giving Republicans a huge pickup opportunity in a heavily conservative House district.

Matheson, one of the last surviving members of the once-powerful centrist Blue Dog Democratic Coalition, had been gearing up for reelection, and the retirement comes as a surprise.

"When I launched my first campaign in 1999, I knew that the arc of my public service would have many chapters. It has been a tremendous privilege to serve the people of Utah during my time in the United States House of Representatives, but my time in the House should not be the sum total of my service," he says in a statement posted to his Facebook page. "Today, I am announcing that I will not seek reelection to the House of Representatives."

Matheson represents the most Republican district of any Democrat in Congress — Mitt Romney won it with 67 percent of the vote — and Saratoga Springs Mayor Mia Love (R), who nearly beat Matheson in 2012, has been pulling in big fundraising totals for a rematch. 

Matheson, the son of a popular former Utah governor, is the only Democrat in Utah's congressional delegation. He has one of the most conservative records of any House Democrat, voting against ObamaCare and for its repeal on multiple occasions and against House Minority Leader Nancy Pelosi (D-Calif.) to be House Speaker in both 2010 and 2012.

The former head of the Blue Dogs' decision to retire is the latest blow to the group, which has shrunk from 54 members before the 2010 election, to 15 currently in Congress.

While Love is the GOP front-runner, the field could grow now that Matheson has retired. If she wins the seat, she'd become the only Republican African American woman in Congress.

This story was updated at 3:00 p.m.