At this stage of the game, that might be more significant than any poll numbers, though in key battleground states, those figures are looking pretty good for Ms. Clinton, too. Now is the point in a presidential election when candidates are building state-by-state organizations to mobilize support and get out the vote in November. And particularly flush campaigns, like President Barack Obama's in 2012, can use the lull between the end of the primaries and the party conventions to begin to define the opponent in the worst possible way. He did that to devastating effect against Mr. Romney in 2012, and Ms. Clinton is doing the same against Mr. Trump now. She and allied groups are up with $23 million worth of ads in battleground states, to Mr. Trump's $0.