Even after Philadelphia, the momentum of the race to the White House points in Trump’s direction. After what even critics said was a highly effective Democratic National Convention, Hillary Clinton appears to have emerged as . . . the underdog. Conventional wisdom has long held that Mrs. Clinton will capture the White House in November, regardless of the challenges her candidacy faces. With three months to go, that calculus looks shaky. The 2016 election is trending toward Donald Trump. Mr. Trump is surging as well in swing states, where he is now slightly ahead, within the margin of error, or...