Here is a small part of a good article about this by Rachel Shorey and Lilia Chang in the New York Times about this new phenomena:
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. The Democrats are not just filing to run in districts where Mrs. Clinton performed well. They are also running for conservative seats that were uncontested in 2016 and where Republicans remain heavy favorites, in states like Texas, Arkansas and Nebraska. . . . Even with the political wind at their backs, most Democratic challengers will be underdogs. “Incumbents have a lot of built-in advantages,” said Michael Beckel of the bipartisan campaign finance reform group Issue One. “They have higher name recognition, and they can spend a lot of time building up a huge campaign war chest to ward off opponents. Additionally, interest groups in Washington are incentivized to bet on incumbents.” But at least the Democrats have the candidates. “If you have a candidate running,” Mr. Beckel said, “your odds of picking up seats increases.”