But Mr. Trump has taken a liking to Mr. Strange, who, he noted at the rally, has not asked for any presidential favors, and several Senate Republicans have told him that Mr. Moore would be an unreliable ally. And should Mr. Strange prevail, the president will have established himself as the dominant force on the right, someone able to transcend the establishment-versus-Tea Party divide that has shaped Republican politics for the last eight years.

“Trump becomes the validator of what the outsider, conservative perspective is,” said Steven Law, who runs the Senate Leadership Fund, a group inspired by Senator Mitch McConnell that is pouring over $10 million into the race on behalf of Mr. Strange. “You had groups on the far right that thought they had the monopoly on that, but they’ve been supplanted by Trump.”…

“If Roy Moore wins, Bannon and all the other of those people will pop out of the woodwork everywhere,” said Senator Richard C. Shelby, the veteran Alabama Republican, referring to the former Trump adviser Stephen K. Bannon, who has taken a leading role against the preferred candidate of his former boss.