The latest from Gallup:
Gallup Daily tracking for the week ending April 4 finds the two major parties tied at 46% in the congressional voting preferences of registered voters nationally. In the two weeks since Congress passed healthcare reform on March 21, Democrats have tied or trailed the Republicans, after having at least a slight advantage in the weeks prior.
Given Republicans' typical voter turnout advantage in midterm elections, even Republican parity with the Democrats in the candidate preferences of registered voters could translate into significant Republican gains on Election Day.
Jim Geraghty notes that anti-incumbent sentiment is higher now than it was right before the 1994 and 2006 elections:
49 percent of registered voters say the representative in their congressional district deserves reelection, and 40 percent say their representative does not. In late October 2006, the split on that was 54 percent "deserves," 33 percent "doesn't deserve." In November 1994, the split on that question was 54 percent "deserves," 30 percent "doesn't deserve."