From Charlie Cook (courtesy of National Journal):

"The bottom line is that at this stage, Republicans should consider themselves lucky if their net losses stay in the 20-25 range in the House, four or five seats in the Senate, and between five and eight governorships. It would be a tough election, losing their majorities in the House and governorships, but it would fall short of the devastating losses that are possible. But the chances of this thing going bigger -- far bigger -- still exist, and there are quite a few veteran Republican strategists, people who have done tons of races in all kinds of states and districts for many years, who are bracing themselves for that distinct possibility."

The analysis piece in its entirety.

Remember: Charlie Cook is the best (and he generally leans Republican).