Donald Trump is trying to quickly reset his presidential campaign to address worsening poll numbers and growing isolation from influential members of the Republican Party.

At weekend rallies, the GOP nominee read from a hand-held script and offered endorsements for the re-elections of a trio of Capitol Hill Republicans whom he had toyed with rebuffing. On Monday, he will head to Detroit to deliver an economic policy address that is expected to draw contrasts with Democratic nominee Hillary Clinton.

Many in Mr. Trump’s party have been clamoring for weeks to see these kinds of adjustments. If he is to persuade Republican skeptics to buy back into his campaign, just weeks before the crucial post-Labor Day stage, the unorthodox, first-time candidate now must show he can make the changes stick.

Part of the issue for the New York businessman is that he has run his campaign much like his family business, with his grown children as his top counselors and surrogates. That has meant he hasn’t developed a strong connective tissue to party stalwarts and activists that can sustain a candidate through difficult times.

Reports of Republicans leaving the party, lining up behind Libertarian candidate Gary Johnsonor even backing Mrs. Clinton gained momentum last week, after the nominee criticized the parents of a U.S. soldier killed in Iraq.