In a Northern Virginia swing district that is one of the most watched in this midterm election, one-quarter of the electorate can’t really say what they think of state Sen. Jennifer Wexton. But in most polls, the Democrat is comfortably ahead of Rep. Barbara Comstock, the two-term Republican who is ubiquitous. That phenomenon says plenty about this particular moment in this particular suburban district across the river from Washington, heavy with college-educated, affluent voters and growing immigrant communities. Voters in Virginia’s 10th Congressional District say they want to vote for a Democrat — any Democrat — as a check on...