Stephen K. Bannon and two of his longtime benefactors are putting together a political coalition designed to ensure that the victory of a Republican insurgent in the Alabama Senate primary this week was just the beginning of the surprises that await the party establishment.

Mr. Bannon brings to the effort the political and promotional skills he showed as President Trump’s chief strategist and advocate for populist stances on issues like immigration and trade. His benefactors, the billionaire hedge fund investor Robert Mercer and his daughter Rebekah Mercer, bring wealth and their own proven anti-establishment streak…

He and Mr. Mercer began hashing out a rough outline for a “shadow party” that would advance Mr. Trump’s America First agenda — even if Mr. Trump himself strayed from it — during a five-hour meeting last month at the family’s Long Island estate a couple of days before Mr. Bannon’s resignation from the White House.