It's been three months since a Georgia special election gave observers the chance to judge President Trump's impact on an unpredictable congressional campaign.
Tuesday night will bring an even more interesting chance.
In Alabama, former state attorney general Luther Strange faces former state Supreme Court judge Roy Moore in a runoff election for the Republican nomination to replace attorney general Jeff Sessions in the Senate.
Strange has Trump's endorsement, and he has occupied the seat since February. But Moore led his opponent in recent surveys, and he appeared to be the more popular candidate among Trump's core supporters. There could be a surprise outcome, as The Washington Post's Michael Scherer wrote, depending on how many voters turn out.
December's general election is easier to predict. Unless the Democrat in the race (that's Doug Jones) can mount a competitive campaign in blood-red Alabama, Tuesday night's winner will go into the general election with an advantage.
But that doesn't make Tuesday's runoff any less important. This race lays bare what some have described as the civil war taking place within the GOP.
Strange is the more conventional Republican candidate, backed by Senate Majority Leader Mitch McConnell (R-Ky.). If he wins, incumbent Republicans on Capitol Hill who are worried about challenges from the right will breathe a sigh of relief.
Moore, by contrast, is one of the most out-there characters on the current political scene, with backing from figures such as Sarah Palin, Fox News host Sean Hannity and former White House chief strategist Stephen K. Bannon. If he wins, McConnell's wing of the Republican Party, and perhaps even McConnell himself, are in for some trouble.
Here are five things to watch Tuesday night as the results roll in, beyond whether Moore pulls out another gun on stage.
1. What's next for McConnell?
If Strange wins, McConnell declares victory. He reassures incumbent senators that the political climate is friendlier to them than they might have thought. And he gains a stronger position from which to consolidate his fragile power as Senate majority leader.
If Moore wins, McConnell finds himself in a tough spot. Incumbent senators will be anxious about the political climate they face. There could be retirements, sooner or later. And McConnell will look more fallible, a leader whose power is waning.
Readers interested in the Senate outlook for 2018 should also watch for reactions from members who might retire, such as Sen. Jeff Flake (R-Ariz.), and members with primary challengers, such as Sen. Dean Heller (R-Nev.).
The way they handle the result, particularly if Moore wins, will say a lot.
2. What's next for Trump (and what will he say or tweet)?
Trump is clearly conflicted about his decision to endorse Strange. First, it can't be easy to see his candidate down in the polls. Second, Moore has the support of the pro-Trump crowd. And third, the president's relationship with McConnell isn't great.
At a rally for the campaign Friday night in Huntsville, Ala., Trump revealed his mixed feelings. "I'll be honest — I might have made a mistake," he said.
If Strange wins Tuesday night, his triumph vindicates Trump, who can forget Moore was ever an option. If Moore wins, Trump says he will back him wholeheartedly, and he might even try to take some of the credit.
3. Who controls the base — Bannon or Trump?
Bannon recently returned to Breitbart News, one of the loudest pro-Trump outlets on the Internet. He has had to thread a careful needle with this runoff, because he represents Trump's base but opposes the guy Trump endorsed.
Once the result is clear, it's worth looking at how Breitbart responds. If Strange wins, that would be a big defeat for Trump's base and insurgent candidates whose brand is "disrupting" Washington. Would Breitbart downplay the importance of the primary in that case? Would it launch attacks against the establishment with renewed energy?
It's not hard to predict how the site will react to a win by Moore. The main headline as of Tuesday afternoon was a quote from Rep. Mo Brooks (R-Ala.) saying a Moore victory would be a "seismic earthquake in Washington, D.C." Bannon is also expected to paint a Moore win as a boon to Trump, though the president endorsed Strange.
"We did not come here to defy Donald Trump," Bannon said at a recent rally for Moore. "We came here to praise and honor him."
4. How does the runoff shape the general election?
It might sound as though Republicans are the only ones interested in the primary's outcome. But Democrats are also waiting for the result, to see how they should direct their resources ahead of the final vote.
If Strange wins, conventional wisdom says Democrat Doug Jones has less of a chance at Sessions's Senate seat. An establishment-leaning Republican in Alabama against a Democrat? You can guess how that would turn out: The state last elected a Democrat to the Senate in 1992.
If Moore wins, on the other hand, Jones gets to run against a candidate whose controversial views might make him easier to beat, someone he could paint as too extreme to represent Alabama before the world. If he's seen to have a fighting chance, Jones could find himself receiving Democratic money from around the country.
5. How many people turn out to vote?
Finally, let's get the numbers question out of the way.
Turnout in Tuesday's election is expected to be low. Only 425,379 people participated in the first round of voting on the Republican side in mid-August, and the second round might only see a slight increase in that number.
On Strange's side, organizations such as the U.S. Chamber of Commerce have made formal efforts to encourage people to turn out to support him. But even if the total number of voters is markedly higher than last month, it is not clear which candidate that will benefit.
The best way to think about it is this: The lower the turnout, the higher the uncertainty about who might win. Moore led Strange in recent polls, but if few people come out to vote, those leads might not mean anything.
The polls close Tuesday at 7 p.m. local time.
Amber Phillips contributed to this report.