AND THEN THERE WERE FIVE…. There are 41 senators in the Republican caucus right now, and looking ahead to 2010, we already know that five of them aren’t sticking around.

Sen. George Voinovich just made it official: He will finish the last two years of this six-year term and then retire from the U.S. Senate after 2010.

This will end more than four decades in politics for the former Ohio legislator, Cleveland mayor, Ohio governor and two-term U.S. senator. It also will open the floodgates for a host of ambitious Ohio politicians who hope to move up.

Voinovich’s announcement comes just a few days after Missouri’s Kit Bond (R) announced his retirement. The Republican Party no doubt hoped to keep retirements to a minimum in order to conserve campaign resources and maximize likely victories. And yet, in addition to Voinovich and Bond, Mel Martinez (Florida) and Sam Brownback (Kansas) are also not seeking re-election, and Kay Bailey Hutchison (Texas) is planning a gubernatorial campaign that will create a fifth Republican open-seat contest.

What’s more, with three more Republican incumbents likely to face major challenges next year — Burr in North Carolina, Gregg in New Hampshire, Specter in Pennsylvania — these announcements make the 2010 cycle that much more difficult for the GOP.*

Sen. Orrin Hatch (R-Utah), an aggressive NRSC fundraiser, told the Politico the recent announcements have “caused some panic” in Republican circles, adding, “I’m really concerned.”

As for Ohio, with Voinovich stepping down, look for former Rep. Rob Portman, former Rep. John Kasich, and former Sen. Mike DeWine to be among the leading Republican candidates. Portman, in particular, has already begun lining up endorsements and financiers, and may have the field to himself if Kasich runs for governor.

On the Democratic side of the aisle, keep an eye on Ohio Secretary of State Jennifer Brunner, Lt. Gov. Lee Fisher, and Reps. Zack Space and Tim Ryan.

* corrected

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.