Charlie Cook says that "absent any national tide" Democrats could lose a dozen or so House seats next year and pick up a seat or two in the Senate.
"Having gained 54 House seats over the past two elections, Democrats now represent 49 districts that GOP presidential nominee John McCain won last year. By comparison, Republicans represent 34 districts that Obama won. Simple arithmetic indicates that in the absence of overwhelming hostility toward the Republican Party, the GOP ought to gain a few, maybe even a dozen or so, House seats."
"On the Senate side, the math is a bit different and is not driven directly by the results of the past two elections. In 2010, Republicans will be defending 19 seats, only one more than Democrats will."