According to RealClearPolitics aggregate polling on the generic congressional ballot, Democrats would win by a large margin if the U.S. 2018 congressional elections were held today.
As made visible by the image provided, Democrats are up +13 on Republicans for a generic house race. Of course, there are many other factors that will play into house races in the country, such as the quality of all the candidates running in each race, the amount each candidate raises, and the incumbent boosts; however, this general poll allows us to speculate the general attitudes of the country as they prepare to vote in 2018.
This poll data is a stark difference from the polls leading up to the 2016 congressional elections, shown below
The aggregate polling data had Democrats winning the congressional vote by .6% and the final results showed Democrats losing to Republicans by 1.1% of the vote.
Even if we take away 1.7% from the current 13 point lead the Democrats currently hold, the results would bring them a resounding victory in the house elections, putting them in a great position to take back the house.
This information coupled with the recent Democratic gubernatorial victories in Virginia and New Jersey, the victory of Doug Jones's Senate race in Alabama, and a general trend of Democrats outperforming their 2016 results by a significant margin play into this perceived victory in 2018. Adding to this is the historic trend in American politics in which the party in opposition to the President performs better in off-year elections, as represented in 2006, 2010, and 2014.
Opinions and different interpretations in the comments are encouraged!