By Erin McPike - November 5, 2010

Republicans have long counted on 2012 as the year to reverse the gains Democrats made in the Senate in 2006, but they didn't expect two years ago that the 2010 elections would turn out as well as they did.

And of all of the competitive races Democrats worried about toward the end of the 2010 cycle, they won the two that most worried them at the beginning - those of Colorado Sen. Michael Bennet and Nevada Sen. Harry Reid, the majority leader.

In 2012, both parties have a handful of incumbents they have to watch carefully; the Democrats just have more that are vulnerable - at least for now, in the current political climate.

Some of the most interesting races to watch may be in the northeast, when Massachusetts GOP Sen. Scott Brown and Connecticut independent Sen. Joe Lieberman are up for re-election.

Brown could face a challenge on the right, but Democrats are certain to make a run at Brown in the deep blue state in a presidential year. Democratic Rep. Ed Markey has been mentioned as a top potential candidate, as have fellow Democratic Reps. Mike Capuano and Stephen Lynch and former Rep. Martin Meehan.

In Connecticut, the independent incumbent lost a Democratic primary challenge in 2006 to the more liberal Ned Lamont, but went on to win the general election with big Republican support. Democratic Rep. Chris Murphy won re-election against Republican state Sen. Sam Caligiuri on Tuesday, and Democrats privately vow that they will target Lieberman in the coming cycle - with Murphy as the most likely contender.

Democrats have to defend 21 Senate seats in 2012, and seven of them are in Midwestern states where Republicans scored major statewide gains on Tuesday. The majority has additional seats to defend in other tough, right-leaning battlegrounds like Montana, Virginia and West Virginia. They must also defend Democratic Sen. Bob Casey in Pennsylvania, where Republicans flipped both a Senate seat and the governor's mansion on Tuesday, and Bill Nelson in Florida, where Republicans also scored critical victories this week.

One of the most visible memebers of the newer classes of Democrats in the Senate is Missouri's Claire McCaskill, a leading advocate of President Obama's during the 2008 Democratic presidential primary.

Republicans view McCaskill as one of their top targets, as she beat GOP former Sen. Jim Talent by just two percentage points in 2006. Talent is eyeing a rematch, his former aides say, and would likely be the candidate to beat. Other potential Republican contenders include Lt. Gov. Peter Kinder and former state Treasurer Sarah Steelman. Strategists expect Kinder is more likely to challenge Democratic Gov. Jay Nixon.

In Montana, Democratic Sen. Jon Tester may get a challenge from Republican Rep. Denny Rehberg, who former National Republican Senatorial Committee Chairman John Ensign tried unsuccessfully to recruit against Democratic Sen. Max Baucus in 2008.

And after the last two years in Minnesota, Ohio and New Jersey, respectively, first-term Sens. Amy Klobuchar, Sherrod Brown and Robert Menendez can expect to serious GOP challenges, too.

Michigan Democratic Sen. Debbie Stabenow may also have a more difficult road ahead as she seeks a third term; Republican Rick Snyder flipped the governor's mansion to the GOP with an 18-point margin over his Democratic opponent.

And Nebraska Democratic Sen. Ben Nelson will be a top target for Republicans, although GOP Gov. Dave Heineman said Thursday that he would not run.

West Virginia Gov. Joe Manchin, who was elected Tuesday after a difficult campaign, is on the ballot for a full term in the Senate two years from now.

A few Democratic senators are potential candidates for retirements next year, and some of the ones to watch include California Sen. Dianne Feinstein, Delaware Sen. Tom Carper, Hawaii Sen. Daniel Akaka, New Mexico Sen. Jeff Bingaman, North Dakota Sen. Kent Conrad, Virginia Sen. Jim Webb and Wisconsin Sen. Herb Kohl.

Should Carper retire, Attorney General Beau Biden is the likely Democratic contender in line to succeed him. And in Wisconsin, Republican Rep. Paul Ryan is said to be mulling a Senate bid, as is House Minority Whip Eric Cantor in Virginia.

New Mexico Democratic Rep. Martin Heinrich would be a likely candidate to pursue Bingaman's seat if he chose to retire, but Heinrich told RealClearPolitics last month that he's certain Bingaman is seeking re-election.

In another Democratic-held seat, after Democratic Sen. Patty Murray survived a difficult re-election against Republican Dino Rossi this year, fellow Washington Sen. Maria Cantwell may be able to breathe easier in her own re-election race in 2012.

Most Republican seats other than Brown's seem to be in territory that is heavily favorable to Republicans, such as Arizona, Indiana, Utah, Wyoming, Mississippi, Tennessee and Texas.

Republicans may have to watch their prospects in Maine and Nevada. Maine Republican Sen. Olympia Snowe is up for re-election but could decide to retire or even face a primary challenge from her right. Nevada Republican Sen. John Ensign is also a wildcard for Republicans. He admitted to having an affair with a campaign aide last year and is under federal investigation for possible criminal violations in covering it up, and Democrats view his seat as a top pick-up opportunity.

Indiana Republican Sen. Richard Lugar is another retirement possibility, as is Texas GOP Sen. Kay Bailey Hutchison.

Other Republicans may face primary challenges, and Utah Republican Sen. Orrin Hatch is already trying to avoid the fate suffered this year by his colleague this year, Bob Bennett. One Republican unlikely to have such a challenge is Senate Minority Whip Jon Kyl in Arizona, although the Arizona Democratic Party maintains "he is always a target."