These charts are from a new Public Policy Polling survey -- done on April 17th and 18th of a random national sample of 648 registered voters, with a 3.9 point margin of error.
In the generic congressional vote (top chart), it shows voters preferring Democrats over Republicans by a 6 point margin (47% to 41%). Of course, it's a year and a half until the next election and a lot could change. But right now the voters don't like what they're seeing in the Republican-dominated 115th Congress.
Perhaps more important, the Democratic voters are more excited about voting in 2018 -- by an 11 point margin over Republican voters (bottom chart). That could also change, but I doubt it will. The Democrats are looking to clip Trump's wings, and the best way to do that is to defeat some congressional Republicans in 2018.
