WASHINGTON — Representative Pete Sessions, a veteran Republican, was re-elected to his affluent Dallas-area House seat in 2016 with 71 percent of the vote, the remaining 29 percent split between the Libertarian and Green Party candidates.

Hillary Clinton won the district by three percentage points, but no Democratic candidate even showed up to ride her coattails.

In 2018, there will be 10.

Federal Election Commission filings show that if a wave crashes on the Republican House majority in November, as many have predicted, Democratic surfers will be on their boards to catch it. Nearly a year out from the election, Democratic candidates have filed in all but 20 House districts held by Republicans. By comparison, Democrats in 80 districts do not have a Republican opponent for their seat.