This was a week of gloomy midterm polls for the Republican Party, with a wave of results projecting a Democratic takeover of the House and maybe even the Senate. But not all polls are created equal. If Republicans bother to read just one, it should be a yet-unreleased survey that tells a more nuanced story.

The data come courtesy of the Club for Growth, a conservative outfit that plays to win. The club’s donors expect it to place smart bets in elections, which it can’t do if it relies on feel-good data. It uses WPAi, the data...