I've written about some of the individual races, but in this post I want to back up and look at the situation as a whole.
There are currently 59 Democrats and 40 Republicans in the Senate, with one Massachusetts seat vacant. Massachusetts will fill that seat by special election on January 19, 2010, and a Democrat will probably win that one.
Senator Kay Bailey Hutchison, Republican of Texas, is expected to resign her Senate seat, in order to concentrate on her candidacy for governor. As I described here, that would also trigger a special election that will be out of the way before the November elections.
So, the expectation is that, going into the November 2010 elections, the split will be 60 Democrats and 40 Republicans.
Republicans seem to have regained momentum, so the question on many people's minds right now is whether they can make significant gains in the mid-term congressional elections.
Some of the biggest gains by Republicans have been 1994, when they gained nine seats, 1980, a gain of 12, and 1946, a 13-seat pickup. Large Democratic gains have included 2008 (eight seats), 1986 (eight seats), and 1958 (13 seats in the existing 48 states, plus three in the new states of Alaska and Hawaii).
While the Republicans may well make gains next year, I don't think they can become the majority party again, quite so soon. They would need to make a net gain of 11, for a total of 51 (if it were 50-50, Vice President Biden would decide the matter in favor of the Democrats). As the figures cited above show, such a gain would not be unprecedented.
But taking a quick look at the individual races (without getting very scientific and looking at polls, the identity of potential challengers, etc.), I identify only seven states where a Republican gain looks at all likely:
New York (Gillibrand)
And here are six, where Democrats could take over Republican seats:
The Republicans' best bet is to try to string together cumulative gains in consecutive election cycles. That's what the Democrats did in 2006 and 2008, with a total gain of 14. Republicans picked up 15 seats in 1978 and 1980. And, further back in history, in the mother of all winning streaks, the Democrats went from a 39 to 56 deficit at the time of Herbert Hoover's landslide presidential victory of 1928, to a 76 to 16 majority, eight years later. Democratic gains (not including left-wing third parties) were eight in 1930, 12 in 1932, 10 in 1934, and seven in 1936.