The Virginia general elections of November 2017 are a case in point. The media takeaway was that the Democrats won all three statewide races and picked up 15 seats in the Virginia House of Delegates. But virtually all of those seats had been carried by Hillary Clinton the previous year. Moreover, Republican candidates ran ahead of President Trump’s percentage in 25 of the 26 closest races. Non-incumbents ran about 2.5 percentage points ahead of the president, while incumbents ran over four points ahead. If Republican candidates did this well in the fall, Democratic gains would be well short of enough to take over the House.

This is the opposite of what has happened in special congressional elections since Mr. Trump’s inauguration. With two exceptions, the Republican candidate has run between five and nine percentage points behind Mr. Trump’s results. One exception, the special election for Third Congressional District in Utah, also should not give Republicans comfort. While the Republican winner John Curtis ran well ahead of Mr. Trump’s percentage, that number was artificially low because of the candidacy of the conservative Mormon independent Evan McMullin. Mr. Curtis ran 19 percentage points behind Mitt Romney’s percentage in 2012; even when the votes from two conservative minor party candidates are added, the combined Republican-conservative vote was seven percentage points lower than Mr. Romney’s.