COLUMBUS, Ohio — Sen. George Voinovich said Monday he won't seek re-election in 2010, becoming the fourth Republican to make departure plans since the Democratic landslide in November.

The 72-year-old Voinovich said challenges facing Ohio and the country will require him to devote all his energy to serving out his current term. Campaigning and fundraising would take time away from his work, he said.

Voinovich's decision was more bad news for Republicans. Democrats, who are just shy of a filibuster-proof margin in the Senate, will be on the offensive in the races for four open seats.

In recent weeks, Republican Sens. Kit Bond of Missouri, Mel Martinez of Florida and Sam Brownback of Kansas have all announced plans to retire in 2010.

Voinovich discounted speculation that his decision not to run was made amid a political environment in Ohio that had turned against the GOP. He's considered a moderate whose support of a proposed federal bailout for the auto industry put him at odds with a significant portion of the GOP caucus.

"The real issue is my wife, my family and the fact that I want to give everything I've got in the next 2 years to see if I can't make a difference and get us out of this terrible situation," he said at a news conference in Washington.

Voinovich said celebrating his wife's birthday recently made them consider their ages and their health, which, while relatively good, can't be taken for granted.

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"We just looked at it and said we need to think about ourselves and we need to think about our family," he said.

Voinovich served as mayor of Cleveland and was a two-term governor before his election to the Senate in 1998. Recent polls have shown his support faltering and Democrats had viewed him as vulnerable in 2010.

Voinovich is known for speaking his mind. He released a strong statement following the auto bailout's demise in Congress, saying the politics of his colleagues had "failed" the industry.

The decision by Voinovich set in motion jockeying in both parties to replace Ohio's senior senator.

Former U.S. Rep. Rob Portman of Cincinnati, who also served as budget director and trade representative under President George W. Bush, plans to run for the seat, a GOP official told The Associated Press on Monday. The official requested anonymity so as not to pre-empt Portman's forthcoming announcement.

Former U.S. Sen. Mike DeWine also is believed to be a possible Republican candidate. Former Ohio Secretary of State Ken Blackwell told The Associated Press Monday he had no interest in the Senate seat and is "singularly focused" on becoming the GOP's next national chairman.

Lt. Gov. Lee Fisher and U.S. Reps. Tim Ryan and Zack Space are potential Democratic Senate candidates.

Portman did not address his own plans in a statement praising Voinovich on Monday. "His countless contributions to our state and to our nation will be felt for many years to come," he said.

Ohio Republican Party Chairman Bob Bennett said Portman told him over the weekend that he was making calls and "feeling things out."

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Associated Press Writer Sam Hananel in Washington contributed to this report.