They called him a madman, a con man, a cancer and worse during an ugly nominating fight, but in the week since Donald J. Trump became the presumptive Republican presidential nominee, many of his former rivals and members of the party’s establishment are already softening on him.

Hard feelings are often short-lived in presidential politics, but the personal nature of this year’s race raised fears that unity would be hard to achieve. While a battered former Gov. Jeb Bush of Florida and a bruised Senator Ted Cruz of Texas are still keeping their distance from Mr. Trump, the recent outreach among others who assailed him for months has been swift and striking.