Regardless of what’s going on in the presidential race, the Senate promises to give the Dems some serious headaches in 2012 : For the first time in two cycles, Democrats will have more seats up for grabs than the Republicans, and the party could see its shrunken majority erased altogether. Several of the senators up for reelection came in on the 2006 Democratic wave, when the party picked up six GOP seats and won control of the chamber. Sens. Bob Casey Jr. (D-Pa.), Claire McCaskill (D-Mo.), Sherrod Brown (D-Ohio), Sheldon Whitehouse (D-R.I.), Jon Tester (D-Mont.) and Jim Webb (D-Va.) defeated GOP incumbents that year but will have to win reelection in 2012. And two senators who won special elections Tuesday, Joe Manchin (D-W.Va.) and Kirsten Gillibrand (N.Y.), will face voters again in two years. Democrats lost at least six Senate seats Tuesday, with results in Washington and Alaska undetermined as of press time, but they retained control. That could change in two years, when Democrats have 21 seats up for grabs, compared to only 10 for Republicans. Also up for reelection are Sens. Joe Lieberman (Conn.) and Bernie Sanders (Vt.), the two Independents who caucus with Democrats — meaning the party has a total of 23 seats to defend. Webb can probably win in VA, and Whitehouse and Sanders are probably safe, but Casey, McCaskill, Brown and Tester will be looking at trouble. And if Manchin ends up being another rubber stamp for Harry Reid, he will not get reelected. Lieberman will probably get another challenge from the left, so his reelection as an independent is no sure thing. To some extent the success of the GOP in 2012 will depend on who’s at the top of the ticket.