Sen. Ben Nighthorse Campbell (search), an outspoken Democrat-turned-Republican, announced Wednesday that he will not seek a third term this fall, citing concerns about his health.

"After a great deal of soul searching and reflection I have decided not to seek re-election," Campbell said in a statement. "I feel the time has come to pass that duty on to another and return to my ranch with my family that I love."

Campbell, 70, has faced questions about his health since he was treated for prostate cancer (search). Last week, he was examined in a Washington hospital after experiencing mild chest pains (search) that turned out to be heartburn.

"After spending another night in the hospital, I realize the deteriorating health may hamper my ability to serve," Campbell said. "Doctors have assured me that after treatment for prostate cancer, the recovery rate is 98 percent. But I believe Coloradans deserve a 100 percent guarantee of service."

Campbell, the only American Indian in the Senate, had spent weeks promising to mount a vigorous campaign. He even put together a recent statewide tour and just Tuesday leveled a blast at one of the four Democrats seeking to replace him.

The Democrats have had trouble finding a big-name candidate to challenge Campbell. Former Sen. Gary Hart and Rep. Mark Udall both declined to run, leaving the party with wealthy think tank founder Rutt Bridges, little-known attorneys Brad Freedberg and Larry Johnson, and educator Mike Miles.