As Mr. Trump has prepared to embark on a difficult fight for re-election, a small but ferocious operation within his campaign has helped install loyal allies atop the most significant state parties and urged them to speak up loudly to discourage conservative criticism of Mr. Trump. The campaign has dispatched aides to state party conclaves, Republican executive committee meetings and fund-raising dinners, all with the aim of ensuring the delegates at next year’s convention in Charlotte, N.C., are utterly committed to Mr. Trump…

“I’ve had probably 10 conversations with the Trump team about the delegate selection process in Florida,” Mr. Gruters said, adding of a potential Republican primary battle, “The base of the party loves our president, and if anybody runs against him, they are going to get absolutely smashed.”

State and local Republican organizations typically operate below the radar of national politics, but they can be vital to the success of a presidential candidate. Party chairmen and their deputies are tasked with everything from raising money to deploying volunteers to knock on doors, and in many states they help choose delegates for the nominating convention.