Analysis of the most recent poll data from Nate Silver:
. . . the poll that Gallup published Friday ought to concern Republicans. It shows a 7-point Democratic advantage on the generic Congressional ballot — meaning simply that more Americans told Gallup they plan to vote for a Democrat for Congress next year. Although the generic ballot is a crude measure, it is probably the best macro-level indicator of the direction that the House is headed in.
Last year, Republicans won the popular vote for the U.S. House — essentially what the generic ballot is trying to measure — by 7 percentage points. So a poll showing Democrats 7 points ahead instead is a pretty significant swing.