In part for that reason, Alabama’s election results are among the most stable in the country. Since 1984, Alabama has swung less between presidential elections than any other state, moving an average of just 4.8 points from the prior presidential result. Alabama also has the third-lowest average share of third-party votes in the country in presidential elections since 1980, which may be a rough measure of the strength of partisan allegiances.

A reason that Alabama might be tougher still for Democrats is that its staunchly conservative white voters are particularly tough to persuade. White evangelical Christians represent about half of Alabama’s electorate, according to exit polls, and more than 90 percent of them probably supported Mr. Trump.

There’s no way to be sure that it’s harder to persuade a white evangelical Christian in the racially polarized South than a less religious, white voter in the North. But almost all of the states that have elected senators against the grain of a state’s partisan lean in the last decade or so look nothing like Alabama. Those states — like Montana, North Dakota, Nebraska, Massachusetts, Alaska and West Virginia — are among the whitest in the country. All have below-average levels of evangelical Christians, and several have a recent track record of voting for the other party in statewide contests. These same white, Northern, less religious states always top the list of the highest tallies for third-party candidates, and usually have above-average swings in presidential elections as well.

It’s not easy to come up with recent favorable precedents for Democratic victories in the Deep South. Perhaps the best involves David Vitter, a Louisiana Republican who managed to lose the governor’s race by 12 points in 2015. Mr. Obama lost Louisiana by 17 points in 2012. Mr. Vitter was dogged by a prostitution scandal from nearly a decade earlier.

Another promising precedent for Democrats happens to be Mr. Moore himself. He won by only four points in his 2012 campaign for Alabama chief justice, and that was without the sexual harassment allegations that have shaken his current Senate campaign. It was the worst performance by an Alabama Republican running for statewide office since 2008.

Black voters represented a larger share of the election in both of these contests than they have in the post-Obama era, so one might assume that Democrats would fare a couple of points worse with today’s turnout patterns.

Even so, Democrats fared well even though neither Mr. Vitter nor Mr. Moore in 2012 was as weak as Mr. Moore is today. Now, national political conditions are plainly more favorable for the Democrats. And this is a special election, when surprising results are a little more common.