In 2015, Donald Trump burst on the scene with a megaphone, a populist message, an army of grassroots supporters — and not much else. If he runs for the Republican nomination again, as seems likely, it will be different. Early in his first presidential campaign, he had to be grateful for every small crumb of support from Republican officialdom — an endorsement from then-U.S. Sen. Jeff Sessions, or former New Jersey Gov. Chris Christie. Now, Trump is a former president of United States who looms incredibly large over his party. He and his loyalists have forged a new establishment —...