Rep. Mike Ross, D-Ark., will not seek reelection in 2012, according to a senior source, becoming the fourth moderate Blue Dog Democrat to head to the exits this year after a midterm election cycle that decimated their ranks.

Ross was first elected in 2000 and is now Arkansas’ lone Democrat in the four-member House delegation. Ross’s retirement makes his House seat a prime battleground for 2012, one that Republicans believe they can pick up. The current district favored Republicans at the national level, voting 58 percent for John McCain in 2008. Under the state’s already-approved redistricting plan, it becomes slightly more Republican.

Despite his district’s preference for Republican presidential candidates, Ross won consistently by comfortable margins. He didn’t even face Republican opposition in two of the last four elections, a sign of his popularity. But he won 57 percent last year, his lowest total since his first re-election.

Ross was singled out by National Republican Congressional Committee Chairman Pete Sessions and NRCC Vice Chair Greg Walden at a briefing last week as one of 12 House seats they believed they could defeat in 2012. The NRCC has also already run a radio ad in April against Ross after he didn’t vote in favor of a budget.