Mr. Trump already knows something about the limits of his influence in Alabama politics. He and the country’s top Republicans repeatedly endorsed Senator Luther Strange, Mr. Moore’s leading rival, in their primary election, only to see Mr. Strange lose by nine percentage points.

Mr. Trump then promptly endorsed Mr. Moore, whom he called “a really great guy who ran a fantastic race.” But the misconduct allegations, which Mr. Moore has denied, have left the campaign gasping and underfunded in the weeks before the Dec. 12 election.

And Mr. Trump may not do much else to aid Mr. Moore. The White House signaled on Monday that Mr. Trump will not use one of the most potent symbols of presidential power — Air Force One touching down somewhere along the campaign trail — to help Mr. Moore. The White House press secretary, Sarah Huckabee Sanders, said that Mr. Trump “is not planning any trip to Alabama at this time, and, frankly, his schedule doesn’t permit him doing anything between now and Election Day.”

The White House’s approach has left Mr. Moore’s campaign with only so much of a political tailwind, even in a state where Mr. Trump won 62 percent of the vote last year.

Abandoned by most of the country’s leading Republicans, except for Mr. Trump, Mr. Moore retreated on Monday night to a rural pocket of Alabama to make his case to voters.

In a speech that was occasionally homespun and sweepingly defiant, Mr. Moore made few references to Mr. Trump, but he…

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