The chart above is from RealClearPolitics. It shows the generic ballot polling of all the most recent polls. Those polls show Democrats with a lead of between 5 and 11 points. The RCP average of all the polls gives Democrats a 7.5 point advantage.
And here is a small part of the latest Cook Political Report:
The bad news for Republicans, of course, is that Trump's approval rating is still 40 percent and that they still trail Democrats on the generic ballot by eight points. That's enough to offset the GOP's edge from favorably drawn districts and endanger their 23-seat majority (by our estimate, Democrats would need to win seven to eight percent more votes for House to win 218 of 435 seats). Moreover, in a reversal from the 2014 midterms, Democrats enjoy a wide voter enthusiasm gap. According to a new CNN/SSRS survey, 51 percent of Democrats and Democratic-leaners said they were "extremely" or "very" enthusiastic about voting in November compared to 36 percent of Republicans/GOP leaners. Young voters, Trump's weakest age segment, also express far more interest in casting ballots than they did four years ago. . . . Retirements are also aiding House Democrats' path to a majority. There are 36 districts where Republicans not running for reelection in 2016, including 12 at serious risk of falling to Democrats (Lean Republican or more vulnerable). Only 18 Democrats are exiting, and just four represent seats at serious risk of falling to the GOP. . . . Our latest ratings feature 55 competitive seats (Toss Up or Lean Democratic/Republican), including 50 currently held by Republicans and five held by Democrats. There are also three non-competitive seats poised to switch parties thanks to Pennsylvania's new map (PA-05 and PA-06 to Democrats, PA-14 to Republicans). Overall, Democrats would need to win 27 of the 55 competitive races to win a majority. We continue to view Democrats the slight favorites for House control.
