With the House Republican leadership awash in concern and with the party’s legislators facing the cratering of their poll numbers, Donald J. Trump centered his attention Tuesday night on the latest WikiLeaks disclosures from a hacking of emails from Hillary Clinton’s campaign.

Using a teleprompter, Mr. Trump told a rally in Panama City Beach, Fla., that “WikiLeaks has given us a window into the secret corridors of government power,” and he ticked through a list of topics that the emails covered.

They included excerpts from Mrs. Clinton’s paid speeches before she had declared her candidacy, such as a line to a Brazilian bank about favoring “open borders” — a statement that plays into Mr. Trump’s line of attack against her immigration position.

It was a reminder of what, in part, made him effective in the Republican primaries against 16 other candidates, lobbing attacks himself instead of leaving it to aides or surrogates.