OCALA, Fla.— broadened to all of Capitol Hill, the Justice Department and the Federal Bureau of Investigation on Wednesday, as the presidential nominees adjusted to a changed political landscape in the waning weeks of the campaign.

With just one more presidential debate before Election Day, Mr. Trump has few high-profile opportunities to gain broad momentum. And while his campaign plans to ratchet up its television advertising, Hillary Clinton’s yearlong investment in a sophisticated ground game to drive her supporters to...