Rural white voters are key for Moore.

If energizing African-Americans is key for Mr. Jones, it is equally crucial for Mr. Moore to get a strong turnout from his longtime base of rural white voters. If these voters decide to stay home because of the sexual misconduct accusations — these conservatives are highly unlikely to cross party lines — it would greatly complicate the Republican’s electoral math.

These small-population counties, stretching across the state’s northern tier and just above the Gulf Coast, are likely to report early. Mr. Moore’s margins in them will go a long way toward indicating whether he can withstand Mr. Jones’s expected success, reported later in the night, in Alabama’s cities.

If Mr. Moore is hitting the sort of marks he reached in his Republican runoff victory in September, over 60 percent in a number of rural counties, he will most likely claim victory.

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If write-in candidates do well, it might not take 50 percent to claim victory.

It is quite difficult for an Alabama Democrat to capture over 50 percent of the vote. But Mr. Jones may not have to capture a majority to win Tuesday. Senator Richard C. Shelby, a Republican and the state’s longest-serving lawmaker, used a national television interview Sunday to remind Alabamians that he had written in the name of another Republican rather than supporting Mr. Moore. His example could spur others in the party to do the same.

And there are two ready options. Lee Busby, a Republican and a retired Marine colonel from Tuscaloosa, has announced a write-in bid. And a number of votes will almost certainly go to someone widely considered the most important man in the state, the University of Alabama football coach Nick Saban.

The more Republicans or independents who write in the name of a third candidate, the lower the threshold Mr. Jones needs to reach. Depending on the number of write-ins, he could potentially win even if he only captures 48 percent of the vote.