Looking ahead to the 2010 Senate races, when Democrats hope to expand their 60-seat supermajority and Republicans want to chip away at it, there are a whole bunch of competitive races on each side -- and with no immediately obvious sure bets for which seats might flip, a whole lot could happen.

In theory, the Democrats could expand their ranks even further, after two consecutive wave elections, for the reason that only one-third of the Senate is up at a given time. Thus, the Senators who are up in 2010 were shielded from the 2006 and 2008 Democratic waves, in which the Dems won nearly all the Senate races they could possibly take. To sweeten the deal for Democrats, more Republican-held seats are up in total than Democratic ones -- because 2004, when this Senate class was last up for election, was a Republican year.

A lot will depend on the national environment. In marginal cases, whether it's a Democratic or Republican year, and the extent of this, can make the difference for the candidates involved, and likely made the difference in some of those close races in 2004, 2006 and 2008.

Let's take a look at some of the top races.

Democratic-Held Seats

Arkansas: Two-term Sen. Blanche Lincoln could be in trouble, in a state that has been trending more and more Republican in federal races. Recent polls have her in dead heats with several lesser-known Republicans: State Sen. Gilbert Baker, state Sen. Kim Hendren, businessman Curtis Coleman, and businessman Tom Cox.

Connecticut: Five-term Sen. Chris Dodd came under fire in 2008 and 2009 for his handling of the financial crisis, and suspicion surrounding a mortgage he took out. Polls have shown him trailing various GOP opponents, though there has been some recent improvement as he works to restore his standing in an anti-incumbent environment. There is a wide GOP field to oppose him: Former Rep. Rob Simmons, former WWE CEO Linda McMahon, stockbroker and financial pundit Peter Schiff, former Ambassador to Ireland Tom Foley, and state Sen. Sam Caligiuri.

Delaware: Sen. Ted Kaufman, who was appointed to Vice President Biden's former Senate seat, is not seeking a full term. Republicans scored a top-level recruit with Rep. Mike Castle, who already represents the whole state in Congress and has been a fixture of its politics for decades. Polls have shown him narrowly leading state Attorney General Beau Biden, a son of Joe Biden, who is widely expected to run. This should be a close one.

Illinois: Sen. Roland Burris, who was controversially appointed to President Obama's former Senate seat last year by the soon-to-be-impeached Gov. Rod Blagojevich, is not seeking a full term. Republicans have been coalescing around Rep. Mark Kirk, who has succeeded in a Democratic-leaning area of the Chicago suburbs. The Democratic primary is between state Treasurer Alexi Giannoulias, former Chicago Inspector General David Hoffman, and Chicago Urban League President Cheryle Jackson.

Nevada: Senate Majority Leader Harry Reid is a top target of Republicans in this swing state. A bad economy and an anti-incumbent mood have contributed to putting him behind two different GOP challengers, former state GOP chair Sue Lowden and former UNLV basketball player Danny Tarkanian. Reid is already advertising on Tv, touting his accomplishments for the state.

Pennsylvania: Five-term Sen. Arlen Specter dramatically switched parties this past April, from the Republicans to the Democrats, due to the threat of defeat in the Republican primary against the more conservative former Rep. Pat Toomey, who nearly defeated Specter back in the 2004 primary. Specter now faces a challenge in the Democratic primary instead, from the more liberal Rep. Joe Sestak. The polls have shown that either one of the two Dems would be in a close race against Toomey in the general election.

Republican-Held Seats

Florida: One-term Sen. Mel Martinez first announced in December 2008 that he wouldn't be running for another term, and this past fall he resigned early and joined a lobbying firm. Republican Sen. George LeMieux was appointed to the seat by Gov. Charlie Crist -- himself a candidate and the GOP establishment's favorite for the seat -- and is not seeking a full term. Crist, a moderate, must fight out a primary with the more conservative state House Speaker Marco Rubio, who has the backing of the Club For Growth. The likely Democratic nominee is Rep. Kendrick Meek. Most races in Florida are close, and this one probably won't be much different.

Kentucky: Two-term Sen. Jim Bunning is retiring, after having accused his party leadership of undermining his fundraising. There are primaries on both sides in this Republican-leaning state. The Republican race pits Secretary of State Trey Grayson against Rand Paul, an ophthalmologist and son of Rep. Ron Paul (R-TX). The Democratic race is between Lt. Gov. Dan Mongiardo and state Attorney General Jack Conway. The polls in this race have been close.

Louisiana: First-term Sen. David Vitter, a strong champion of social conservatism, found his career mired in a 2007 prostitution scandal. However, he has continued on, touting his strongly conservative positions in a Southern state. His opponent is Democratic Rep. Charlie Melancon, a Blue Dog who has voted against the health care bill and against cap and trade, but did vote for the stimulus. Polls have shown Vitter ahead of Melancon, who at this point has much lower name recognition.

Missouri: Four-term Sen. Kit Bond is retiring. The Democrats have settled on Secretary of State Robin Carnahan, a daughter of the late Gov. Mel Carnahan and former Sen. Jean Carnahan, while Republicans have gathered around Rep. Roy Blunt, a former House Republican Whip. This is another state that usually has close races, and the polls so far have given Carnahan a narrow lead over Blunt.

New Hampshire: Three-term GOP Sen. Judd Gregg, who was briefly nominated by President Obama to serve as Secretary of Commerce but then backed out, is not running again. Democrats have coalesced around two-term Rep. Paul Hodes as their candidate in a state that has been trending Democratic. The Republican field is made up of former state ATtorney General Kelly Ayotte -- who has led Hodes in recent polls -- and former state board of education chairman Ovide Lamontagne, businessman Jim Bender, and businessman William Binnie.

North Carolina: First-term Sen. Richard Burr is seeking a second term in a state that has become much more Democratic since his original win in 2004, with the state in 2008 voting Democratic for president for the first time since 1976. The current Democratic candidates are SEcretary of State Elaine Marshall, and attorney Kenneth Lewis. Polls have shown that Burr has not made a big impression on voters, with both low approval and low disapproval, and his race could end up hinging on the overall national partisan climate in November 2010.

Ohio: Two-term Sen. George Voinovich is retiring. The Republican primary is between former Rep. Rob Portman, who served as Trade Representative and Budget Director in the George W. Bush administration, and auto dealer Tom Ganley. The Democratic race is between Lt. Gov. Lee Fisher and Secretary of State Jennifer Brunner. Ohio is a perennial swing state, and the outcome here could end up hinging on the political climate in November 2010.