New Mexico Democratic Senator Jeff Bingaman will announce he is retiring today. New Mexico is a Democrat-leaning state. Obama won in 2008 by 15% , after Bush's narrow 1% (6,000 vote margin) victory in 2004. Republican Susanna Martinez was elected Governor in 2010, and might be the strongest GOP candidate for the seat, but it is unlikely she would run for the open Senate seat so soon after taking office.
Heather Wilson, the former Republican Congresswoman is a possibility for the GOP. She ran for and lost the GOP nomination to run against Tom Udall for the open Senate seat of Pete Domenici in 2008. The Republicans picked up 1 of New Mexico's 3 House seats in 2010 with Steve Pearce's victory in the 2nd district. Pearce, the defeated candidate for U.S Senate in 2008 (Udall won by 22%), could, like Wilson, try again.
The two Democratic U.S House members are potential candidates for the Senate seat -- Martin Heinrich, or Ben Ray Lujan. Heinrich's House seat is a bit more competitive than Lujan's. The Democrats will be favored to retain Bingaman's seat, but the GOP now has a chance to win, which would not have been the case against Bingaman, who won with 71% of the vote in 2006.