CLEVELAND — Donald Trump’s selection of Mike Pence to serve as his running mate was a welcome bit of reassurance to a Republican Party still suspicious of its presumptive presidential nominee.

Trump’s unorthodox populism was good enough to win the GOP nomination in a wild, crowded primary. But party insiders — operatives, donors and grassroots activists who man the trenches for the nominee every four years and are an important part of winning campaigns — have been slow to embrace Trump.