"After prayerful consideration and much thought, my wife Janet and I have decided that I will not seek a third term in the U.S. Senate," Voinovich, 72, said in a statement and reported by The Hill.

"I have never seen the country in such perilous circumstances. Not since the Great Depression and the Second World War have we been confronted with such challenges, as a nation and as a world. 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, who is a former Cleveland mayor and Ohio governor, is the fourth Republican to announce plans to leave the Senate, joining Kit Bond of Missouri, Mel Martinez of Florida and Sam Brownback of Kansas.

In addition, Texas Republican Sen. Kay Bailey Hutchison reportedly is considering leaving the Senate for a gubernatorial run.