Ohio Republican Sen. George V. Voinovich , whose political career has spanned more than four decades, Monday became the fourth GOP senator to announce he will not seek re-election in 2010.

Former GOP Rep. Rob Portman, who served as U.S. trade representative and White House budget director under President Bush, may be the first to announce a run for the seat — although he certainly won’t be the last. “We intend to announce Wednesday and Thursday,” a source close to Portman said.

Portman, who left the House more than three years ago, kept open a campaign committee that had more than $1.5 million in cash reserves as of last October.

Voinovich’s decision adds to the burden for Republican campaign officials as they plot a comeback strategy after two terrible campaign cycles that left them with just 41 of the 100 Senate seats.

He joins three other GOP senators who have already said they will call it quits after this Congress: Mel Martinez of Florida, Sam Brownback of Kansas and Christopher S. Bond of Missouri.

The 72-year-old Voinovich initially appeared headed toward a campaign for a third Senate term, as he had reported $2.6 million cash on hand in his Senate campaign account at the end of September. But he reconsidered over recent weeks.

Voinovich, in a statement, said, “Not since the Great Depression and the Second World War have we been confronted with such challenges, as a nation and as a world,” and asserted that these matters require his full attention as a lawmaker.

“These next two years in office, for me, will be the most important years that I have served in my entire political career. I must devote my full time, energy and focus to the job I was elected to do, the job in front of me, which seeking a third term — with the money-raising and campaigning that it would require — would not allow me to do,” Voinovich said.

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