The Ohio Senate race was upended Friday when the Republican front-runner, state Treasurer Josh Mandel, withdrew from the contest in a surprise move that left the GOP primary and shape of the general election uncertain.

Just one month until the Feb. 7 filing deadline, and four months before the May 8 GOP primary, wealthy investment banker Mike Gibbons is the lone Republican standing in the race to challenge incumbent Democratic Sen. Sherrod Brown, a hard scrabble progressive who defeated Mandel six years ago to win a second term. But GOP insiders expected that with conservative favorite Mandel out of the race, several Republicans would consider running, with his exit possibly sparking a crowded and contentious primary.