Sen. George Voinovich’s (R-Ohio) retirement announcement Monday set off a firestorm of speculation in Buckeye State political circles, with many former and current House Members looking at the race to replace him.

The open-seat race is sure to be one of the most competitive — and one of the most expensive — of the 2010 cycle.

Voinovich’s aides had stressed the Senator’s re-election plans until last week, often citing the $2.7 million that he had stored up for his campaign as proof, so would-be successors haven’t had long to mobilize. But at a Capitol Hill news conference Monday afternoon, Voinovich said he wanted to devote the next to years to legislating and his family.

“These next two years will be the most important of my career, and I must address my full attention” toward the Senate, Voinovich said.

Ohio has traditionally been a battleground state, although it has voted for more statewide Democrats in 2006 and 2008. What’s more, Voinovich’s retirement could be key for Senate Democrats to get a filibuster-proof caucus in 2010.

For Republicans, Monday’s speculation surrounded former Rep. Rob Portman (Ohio), who has reportedly already tapped Cincinnati businessman Mercer Reynolds to help him with fundraising for his campaign. Portman stepped down from his 2nd district House seat in 2005 to be U.S. trade representative, followed by a stint as director of the Office of Management and Budget.

Portman and former Rep. John Kasich (R-Ohio), who is looking at a bid for governor, have been traveling in GOP circles around the state and speaking to local Republican activists. What appears to give Portman the early advantage is that he still has $1.5 million in his campaign account from his House races to launch a statewide campaign.

The Democratic Senatorial Campaign Committee took a shot Monday at Portman, who has yet to formally announce a bid, signaling Democrats will do everything that they can to link him to outgoing President George W. Bush.

“It’s jaw-dropping that Republicans would seem to turn to a Washington insider like Rob Portman who was one of the architects of the Bush economic policies that have run up trillions in deficits and shipped jobs overseas,” DSCC spokesman Matthew Miller said.

If Portman does not run, Republicans might turn to former Sen. Mike DeWine (R-Ohio), who lost his seat in 2006 to now-Sen. Sherrod Brown (D-Ohio). In a telephone interview Monday, DeWine declined to discuss any of his personal political ambitions.

“I’m not going to talk about that today,” DeWine said. “This is George Voinovich’s day.”

But according to one well-placed Ohio Republican source, DeWine has his sights set on running for state attorney general in 2010.

In the search for the best possible Buckeye State candidate, Democrats could recruit Lt. Gov. Lee Fisher (D). Fisher, however, was coy about a run in a Monday phone interview.

“My sole focus remains on working with Gov. [Ted] Strickland to respond to this unprecedented national economic crisis and to grow Ohio’s economy,” he said. “I respect and admire Sen. Voinovich’s long and very distinguished record of public service, and I’ll have more to say on a possible run for the U.S. Senate at a later time.”

While several House Members have expressed interest in running for the seat, Fisher could be formidable if the popular Strickland comes out early and endorses his lieutenant governor.

Senate Democrats might look next to Rep. Tim Ryan (D), who confirmed Monday that he is considering a run for the seat.

“I am singularly focused on what’s best for my constituents in Northeast Ohio,” Ryan said in a statement. “As a member of the House Appropriations Committee I am in a very good position to help create and expand important economic development initiatives in the State of Ohio. That said, if I can have a greater impact by being in the Senate, I will certainly consider it.”

Ryan’s critics say he has never had a tough race in his heavily Democratic northeast Ohio district since he won the seat following former Rep. Jim Traficant’s (D-Ohio) expulsion from the House.

Democratic leaders considered Ryan for a statewide run in 2006.

Democrats might also be looking to Rep. Zack Space (D), who represents a much more conservative district than Ryan. His office did not return a request for comment Monday, but sources say that Space has grown increasingly close to Brown since he was elected in 2006 and that Brown could be helpful to him in a statewide run.

Still other Democrats say Rep. Betty Sutton (D) could also look at the race.

“The retirement of George Voinovich offers Ohio Democrats a tremendous opportunity to win and hold a second seat in the U.S. Senate,” Sutton said in a statement. “At this critical time for Ohio and our nation, we must elect a strong leader who can effectively work within the Senate to spur job growth and restore the promise of the middle class.”

All three Democratic House Members, however, do not have nearly as much seed money as Portman. At the end of the last reporting period on Nov. 24, Ryan reported having $297,200 in cash on hand, Space had $284,000 and Sutton had $33,400.

John Stanton contributed to this report.