Florida Sen. Bill Nelson will face voters who elected a Republican governor and kept the other U.S. Senate seat in GOP hands. The same thing happened in Ohio, where Sen. Sherrod Brown will seek a second term. Obama won both those states in 2008, and they will be fiercely contested again.
Sen. Jon Tester of Montana, who narrowly won his first term in 2006, will run again in a state that has gone Republican in all but two presidential elections since 1948.
In Nebraska, Democratic Sen. Ben Nelson likely will seek a third term in a state that Obama lost by 15 percentage points. Nelson, perhaps the Senate's most conservative Democrat, caught a break when GOP Gov. Dave Heineman said he would not run for Senate. But another potentially strong challenger, Nebraska Attorney General Jon Bruning, may jump in.
Democratic Sens. Jim Webb of Virginia and Claire McCaskill of Missouri could face rematches in their bids for a second term in 2012. Republicans scored impressive victories in both those states last week.
Former Virginia Gov. and Sen. George Allen, whom Webb narrowly beat four years ago, might run again. The same goes for former Sen. Jim Talent of Missouri, who lost to McCaskill in a close contest.
Few states are more daunting for Democrats than North Dakota, which last voted for a Democratic presidential nominee in 1964. Nonetheless, Democrat Kent Conrad has won a remarkable five Senate elections there, and presumably will try again in 2012. He's lucky that popular GOP Gov. John Hoeven just grabbed the open seat being vacated by Democratic Sen. Byron Dorgan. But another viable Republican might step up.
Not all the problems fall on Democrats. In Maine, centrist Republican Sen. Olympia Snowe has proven a savvy campaigner. But tea party activists just elevated Paul LePage to the governor's office, and the libertarian-leaning conservatives may try to knock off Snowe in the 2012 Republican primary.
In Nevada, a toss-up state in recent elections, Republican Sen. John Ensign is dogged by a staff-and-sex scandal. Re-election may prove tough for him, and Republicans might seek another nominee.
Perhaps no Senate contest will inspire Democrats more than Massachusetts, where they are burning to take back the seat long held by liberal hero Edward M. Kennedy. The state leans heavily Democratic. But Brown stunned the political establishment in the January special election, and it's possible he has more magic up his Republican sleeve.