The Club for Growth thought it had dented Republican presidential front-runner Donald Trump’s chances in Iowa with ads accusing him of being a false conservative, but the billionaire businessman has surged back to the lead in polls, showing a staying power that’s confounded pundits and fellow candidates. The Club’s battle with Mr. Trump is shaping up as an early test of his campaign with little more than two months to go before Iowa holds its first-in-the-nation caucuses.