WASHINGTON — More than any other elected official, Senator Jeff Sessions of Alabama laid the intellectual foundation for President Trump’s brand of nationalist politics, agitating for a hard line on immigration and trade while most other Republicans were in thrall to the U.S. Chamber of Commerce and Mr. Trump was still firing contestants on “The Apprentice.”

Mr. Sessions, who was sworn in as attorney general on Thursday, was succeeded on the same day in the Senate by the attorney general of Alabama, Luther Strange, a former Washington lobbyist and onetime partner at a white-shoe Birmingham law firm with deep ties to the establishment wing of the Republican Party.

“He’s going to be a mainstream conservative Republican,” Karl Rove, the former strategist for George W. Bush, predicted of Mr. Strange, whom he met in the 1990s when the two worked together on the ferocious campaign for Republican control of the Alabama Supreme Court. “He’s very smart, really hard-working.”

The ascension of Mr. Strange to the seat Mr. Sessions held for 20 years offers a vivid illustration of how, even as Mr. Trump tries to steer the Republican Party toward a more populist orientation on some issues, the institutional party still largely comprises business-aligned Republicans.