Citing too much partisanship in Washington, Maine Sen. Olympia Snowe will not run for re-election, she announced Tuesday.

Snowe, a moderate Republican who was often a key swing vote on partisan issues, was first elected to the U.S. Senate in 1994. She previously represented Maine’s 2nd Congressional District in the U.S. House for 16 years.

“After 33 years in the Congress this was not an easy decision. My husband and I are in good health. We have laid an exceptionally strong foundation for the campaign, and I have no doubt I would have won re-election,” she said in a statement.