In a normal cycle, 33 or 34 Senate seats are up. In the next cycle, the number was slated to be 34 until Delaware's Joe Biden was elected vice president. Biden won a seventh Senate term in November at the same time he won nationwide. Delaware law calls for holding a special election at the time of the state's next general election. Biden's appointed successor, Edward Kaufman, has indicated that he will not run in 2010 for the remaining four years in the term.
Hillary Rodham Clinton's selection as secretary of State put a 36th seat on the line in 2010. Appointed Sen. Kirsten Gillibrand will face voters then for the right to serve until 2012, when Clinton would have been up for re-election.
Meanwhile, Republican Sen. Kay Bailey Hutchison, whose seat would not normally be up again until 2012, has indicated she may step down this year or early next year to challenge Texas Gov. Rick Perry in the Republican primary. If she does, it would trigger a special election for a 37th seat next year.
Of the 36 seats certain to be up next year, the contests for at least six -- including five now held by Republicans -- will be open. The five retiring GOP senators are Christopher (Kit) Bond of Missouri, Sam Brownback of Kansas, Judd Gregg of New Hampshire, Mel Martinez of Florida, and George Voinovich of Ohio. The lone Democratic opening is Biden's old seat.
There is also considerable speculation that a sixth GOP seat might open up in Kentucky, where 77-year-old incumbent Jim Bunning is bristling at the suggestion that he will retire. Party leaders privately say that their odds of holding the seat would be better with a younger, fresher face.
Democrats aren't without worries, though. Besides the open Biden/Kaufman seat, they have three appointed senators to deal with -- Michael Bennet of Colorado, who was picked to succeed Interior Secretary Ken Salazar; Roland Burris, who was chosen by then-Gov. Rod Blagojevich to fill the Illinois seat that President Obama held; and Clinton successor Gillibrand of New York. (The Colorado and Illinois seats would have been up in 2010 anyway.)
In winning the White House, Obama carried all four of those states. Nevertheless, Democrats should be particularly nervous about Bennet and Burris. Bennet is a first-time candidate running in a swing state; Burris has a mixed electoral record statewide with four wins and four losses, and he lost a Chicago mayoral race. Plus, his appointment continues to be controversial because Blagojevich tapped him after being arrested amid allegations that he tried to sell the seat.
Although the overall numbers, 17 Democratic seats versus 19 or 20 Republican seats, are not as unbalanced as they were last year, when Republicans had to defend 23 seats to the Democrats' 12, they put the GOP in a difficult position. Three of the open Republican seats -- those in Florida, New Hampshire, and Ohio -- are in states that Obama carried. A fourth is in a state, Missouri, that he lost by fewer than 4,000 votes out of 2.9 million cast. And a fifth could well be sought by popular Kansas Gov. Kathleen Sebelius, a Democrat, if she is not nominated as Health and Human Services secretary.
In terms of preliminary ratings, Republicans have five seats that are now considered "toss-ups" -- Bunning's and the open seats in Florida, Missouri, New Hampshire, and Ohio. Three more GOP seats are rated "lean Republican" -- those of incumbents David Vitter of Louisiana and Arlen Specter of Pennsylvania, along with the open Kansas seat.
In contrast to the GOP's eight vulnerable seats, the Democrats have only two at the moment: Burris is in the "toss-up" category, and Bennet is "lean Democratic."
Jennifer E. Duffy, Senate editor of The Cook Political Report, contributed to this report.