Tossup: Bill Nelson (Florida), Debbie Stabenow (Mich.), Claire McCaskill (Mo.), Jon Tester (Mont.), Virginia (open; Jim Webb is retiring), Wisconsin (open; Herb Kohl is retiring).

Nelson benefits from not having a truly top-notch opponent, someone like former Gov. Jeb Bush. But he’ll have a better opponent than the one he faced in 2006, Katherine Harris, and the environment will probably be much less favorable than it was that year. Like Sherrod Brown, he leads his opponents by double digits but is under 50 percent. Unlike Brown, he occupies a state that will probably be much less tolerant of his votes on things like health care, and his potential opponents are much more seasoned.

In 2006, Michigan looked like it would be competitive until September or so. Stabenow pulled away against fairly weak competition and won handily. This time, the environment is much worse for Democrats. Her opposition isn’t top-notch, but her approval ratings have been well below 50 percent in a state that swung heavily toward Republicans in 2010.

McCaskill narrowly defeated Sen. Jim Talent in 2006. Since then Missouri has moved toward the Republicans; it is the only true swing state that Obama failed to carry in 2008, and it gave a solid win to a flawed GOP Senate candidate in 2010. McCaskill has led her potential GOP opponents in most polls, but those leads have been in the low single digits.