Ryan was viewed as a capable Democrat who would have been a serious contender for his party's nomination. The only Democrat currently in the race is P.G. Sittenfeld, a less well-known Cincinnati councilman.
The big question continues to be whether former governor Ted Strickland (D) will wage a campaign. Strickland is seen widely as Democrats' best possible option against Portman, a well-funded senator who is already gearing up for reelection in his swing state.
Strickland is expected to announce a decision soon.
Democrats must gain at least four seats to win back the Senate majority in 2016. They see Ohio as a top priority in the battle.