Russ Schriefer, a strategist for Mitt Romney’s 2012 presidential campaign — which raised an average of $633,900 a day during its first reporting period of the 2012 race — noted that previous presidential front-runners had spent months, if not years, carefully considering the timing of their announcements and ensuring that their launches would be paired with strong initial fund-raising reports. Mr. Trump’s start, by contrast, appeared more ad hoc, he said. “The rules that applied to Mitt or Hillary or Jeb just don’t seem to apply to Trump,” Mr. Schriefer said. But inopportune timing would be a peculiar explanation for...