The New York Times Upshot/ Kaiser Family Foundation poll found that many registered voters who said they support the Democrats also said they did not like the Affordable Care Act. If the Republican Party builds successfully on that opposition, it could erase the leads the incumbents now hold.
In Arkansas, for example, Sen. Mark Pryor leads Republican Rep. Tom Cotton by 10 percentage points. But 16 percent of those who said they would vote for Pryor said they disagree with his vote in favor of the health care law.
Pryor currently has the biggest lead among Southern Democrats. In North Carolina, Kay Hagan leads by only 1 point among registered voters and has a 3-point disadvantage among those who say they are likely to vote in November.
Louisiana is hard to call because its system involves an open primary followed by a run-off between the two top vote-getters unless someone gets at least 50 percent of the vote in the first round. Sen. Mary Landrieu has the support of 42 percent of registered voters, far more than any Republican now in the field, but less than she needs to win.
Polls of registered voters tend to overstate the Democrats' advantage, especially in off-year elections and in states where the party depends heavily on young and minority voters.