WASHINGTON - Democrats came up just short of a winning a filibuster-proof majority of 60 in the 100-member Senate in last November’s election.  But they may do it in next year’s contest — thanks largely to a rising number of Senate Republicans calling it quits.

George Voinovich – a highly respected, two-term moderate — on Monday became the fourth Senate Republican to announce he won’t seek reelection in 2010.

Voinovich’s decision followed earlier such announcements by Republican Sens. Sam Brownback of Kansas, Mel Martinez of Florida and Kit Bond of Missouri.

So far, there’s been no official word that any of the Democrats up for reelection next year won’t seek another term — though Vice President-elect Joe Biden’s pending Senate replacement is expected to step aside in 2010. That would open the way for an anticipated run by Biden’s son Beau Biden, Delaware’s attorney general.

Traditionally, Senate incumbents have a number of advantages over challengers, including name recognition and the ability to raise money.

Voinovich, who next year will mark his 74th birthday and 44th year in public office, said it’s time for him to move on.

“I still have the fire in my belly to do the work of our nation, but after serving the next two years, it will be time to step back,” Voinovich said in a statement.

Democrats now have 57 seats in the Senate, and may soon hit 59. A legal challenge is expected to be decided within weeks of Democrat Al Franken’s razor-close victory in Minnesota.

And Democrats hope to soon resolve a dispute over embattled Illinois Gov. Rod Blagojevich’s appointment of Roland Burris to fill the seat vacated by President-Elect Barack Obama.

Democrats have resisted seating Burris because Blagojevich is accused of having earlier tried to sell the seat. But with the blessing of Obama, they seem to be determined to soon reach an “amicable solution.”

Photo Credit: Reuters/Mannie Garcia (Sen. George Voinovich with President George W. Bush in 2004)