Donald J. Trump, who in recent days has mocked a political opponent’s wife, defended a campaign aide arrested on a charge of battery and suggested punishing women who terminate pregnancies, may have surrendered any remaining chance to rally Republicans strongly around him before the party’s July convention in Cleveland.

At a moment when a more traditional front-runner might have sought to smooth over divisions within his party and turn his attention to the general election, Mr. Trump has only intensified his slash-and-burn, no-apologies approach to the campaign.

“He should have started uniting the party in March,” said Henry Barbour, a Republican National Committee member from Mississippi who previously supported Senator Marco Rubio of Florida, “and he is making it harder on himself.”

Republicans who once worried that Mr. Trump might gain overwhelming momentum in the primaries are now becoming preoccupied with a different grim prospect: that Mr. Trump might become a kind of zombie candidate — damaged beyond the point of repair, but too late for any of his rivals to stop him.