Story highlights The key to Democrats winning back the House is expanding the playing field of competitive races

The current pro-Democratic -- or, more accurately, anti-Trump -- environment helps on that front

Washington (CNN) Conditions are right for a major Democratic wave to sweep out the House Republican majority next fall, according to Nathan Gonzales, who runs the "Inside Elections" campaign tipsheet.

"The midterm elections are still nearly a year and a half away, and the political dynamics could yet change, but we shouldn't ignore the fact that history and the current environment are merging together for a potentially great set of elections for Democrats in November 2018," writes Gonzales in a piece published Wednesday morning

To that end, Gonzales is adding nine Republican seats to his list of potentially competitive races and moving the ratings on 10 more GOP-held districts in favor of Democrats. That brings his list of competitive contests to 39 for Republicans as compared to just 14 for Democrats.

As Gonzales notes, two factors argue for a wave election: History and polls.

First, history. In the last 20 midterm elections, the president's party has lost seats in 18. In those 18 elections, the average seat loss is 33. Democrats need 24 seats to re-take the House majority in 2018.

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