CNN

Sources say Sen. George Voinovich, a Republican from Ohio, will retire from the Senate.

WASHINGTON (CNN) - Sources close to Sen. George Voinovich, R-Ohio, tell CNN he will announce Monday he is retiring from the Senate.

The 72-year-old senator and former Ohio governor has been contemplating retirement for a while but made his final decision this weekend in Florida, where he was joined by his wife, children and grandchildren, according to one of the sources.

Voinovich plans to serve out his term, which expires in 2010.

He plans to make the announcement at 4 p.m. ET Monday in Washington, the source said.

Voinovich will be the fourth Republican senator to announce he will not run for re-election in 2010. Sen. Kit Bond, R-Missouri, said late last week that we would retire, joining Sen. Mel Martinez, R-Florida, and Sen. Sam Brownback, R-Kansas, who had already announced they would leave Capitol Hill when their terms end at the close of the 111th Congress.

Republicans must defend 15 incumbents and four open seats in 2010. (Sen. Kay Bailey Hutchison, R-Texas, whose term is not up until 2012, is seriously considering a 2010 gubernatorial bid and could also be vacating her seat early).

Meanwhile, Democrats have to defend 15 incumbents and two seats where special elections are expected to be held. In Delaware, a special election will be scheduled for Vice President-elect Joe Biden's seat as well as a special election in New York should Hillary Clinton be confirmed as the next Secretary of State.