HOUSTON — J. Mark Metts, a 60-year-old partner at one of this city’s prestigious law firms, had never voted for a Democratic presidential candidate until 2016. Now he and some of his neighbors in the moneyed River Oaks enclave of Houston are about to oppose a Republican once again, to register their disapproval of President Trump. “With Congress not really standing up to Trump, this election is becoming a referendum,” Mr. Metts said, explaining why he would no longer support the re-election of Representative John Culberson, an eight-term Republican. Mr. Culberson is now running roughly even with the Democratic candidate,...