Maine Rep. Mike Michaud announced tonight that he will not run to replace retiring Sen. Olympia Snowe (R), a move that all but ensures fellow Democratic Rep. Chellie Pingree will enter the race.

Michaud and Pingree had both moved to begin collecting signatures in the hours after Snowe’s surprise announcement on Tuesday, which came about two weeks before the state’s filing deadline. But Michaud, first elected to Congress in 2002, said he is staying put for now.

“While I have been humbled by all the support and encouragement I have received in the last few days, I’ve decided to not run for the U.S. Senate this year,” he said in a statement. “I want to continue to represent the wonderful people of Maine’s second district and keep working on the unique issues and challenges we face.”

Pingree will likely now be considered the frontrunner in the open-seat Senate race, although the field remains very fluid and she could still face a competitive primary against former Gov. John Baldacci.

In his statement, Michaud seemed to agree with Snowe’s sentiment that partisanship on Capitol Hill had reached an almost unbearable point.

“I join many Mainers in being frustrated with how Washington operates and believe that both sides of Capitol Hill have fallen into a partisan rut,” he said. “However, I am proud of being able to work across the aisle to deliver results and I think, for now, I can best continue those efforts in the House.”

Michaud’s next opportunity to run for Senate will  come in 2014, when Sen. Susan Collins (R) is up for re-election.