The Democratic Party is ahead in generic ballot polls, earning 54.2% of the two-party vote share on average. They hold an 8.4 percentage point lead over the Republicans.
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According to our 20,000 simulations of the 2018 House midterm election, Democrats have a 30.3% chance of taking back the majority on November 6, 2018. Here is one way of visualizing that probability, where each blue rectangle represents a Democratic majority and each red rectangle is a Republican majority:
Source: https://decisiondeskhq.com/data-dives/ddhq-2018-house-midterm-forecast/
Vote next year aka. when this thread is old. Also during this year's remaining special elections.