So what's happening now?

On Tuesday, the long-awaited Republican Alabama Senate runoff finally happens. In one corner we have Sen. Luther Strange, who was appointed to the Senate in January because its previous occupant, Jeff Sessions, left to join the cabinet. In the other, you have Roy Moore, a social conservative firebrand and a former justice on the state supreme court.

Who are these guys?

"Big Luther" Strange (the nickname stems from the fact that he's 6' 9'') is what we'll call the establishment pick. He's the former Alabama attorney general and current incumbent senator, and he's backed by Senate Majority Leader Mitch McConnell, who's seen to it that allied political groups spend mightily in Strange's favor.

Roy Moore is the conservative insurgent. He's twice been the chief justice of the state's supreme court, and he's effectively been kicked off the bench both times – once is 2003 for refusing to remove a statue of the Ten Commandments he'd commissioned on state grounds, and once in 2016 for refusing to recognize gay marriages. This has made Moore a hero to many in Alabama, but his outspoken social conservatism and tendency to buck the GOP establishment has moderate Republicans spooked.

Well, who's going to win?

It's tough to accurately poll special elections, in large part because far fewer people tend to turn up to vote in them than in regular elections. That said, Moore probably has the edge, and currently has a ten-point lead in the RealClearPolitics polling average of the race.

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A big problem for Strange is how he got to the Senate in the first place. Strange was appointed by then Gov. Robert Bentley, who was in hot water at the time. Bentley, a former church deacon who had been caught having an extramarital affair with an aide, was accused of using state resources to keep the relationship under wraps. The state legislature looked ready to impeach him, and the Strange stepped in and asked for time to investigate the matter first.

A couple months later, Bentley appointed Strange to the seat being vacated by Jeff Sessions, leading to rumors of a secret deal between Bentley and Strange. Both deny any deal ever took place, but the whiff of impropriety has haunted Strange from the get-go in this primary.

Where's Trump in all this?

Faced with a choice between a idiosyncratic conservative darling and a more staid establishment choice, President Trump surprised some by going all-in for the latter, and has been heavily promoting Strange. Mr. Trump won Alabama by 29 points and presumably has a fair amount of sway among Republican voters there. However, even as Strange flaunts the endorsement, he still seems to be lagging behind Moore.

Another curious side-note to this drama is how some of Mr. Trump's most prominent backers have bucked the president and saddled with Moore. Former White House strategist Steve Bannon, former Trump aide Sebastian Gorka, Fox News host Sean Hannity, and former Alaska Gov. Sarah Palin have all endorsed Moore over strange. Bizarrely, so has Secretary of Housing and Urban Development Ben Carson.

Alabama, we need you to get our and vote for Judge Roy Moore tomorrow! https://t.co/tN9k3clLEZ — Sarah Palin (@SarahPalinUSA) September 25, 2017

Mr. Trump gets that his candidate might lose, acknowledging during a rally in Alabama on Friday that it might have been a "mistake" to back Strange. Mr. Trump also said he would back Moore should he win the run-off.

Meanwhile, Strange is backed by McConnell, whom Mr. Trump has been feuding with for months. At Friday's rally, Mr. Trump implied that Strange would stand up to McConnell, but that doesn't really make any sense.

Do the Democrats have a shot at winning the Senate seat?

Alabama is about as deep red a state as you can get in many ways, and hasn't sent a Democratic senator to Washington since 1992. (And that senator, Richard Shelby, would become a Republican two years later.) But Democrats at least seem to have a solid option in Doug Jones, a former U.S. attorney. Still, he probably won't win.

What are the stakes?

The first and most obvious takeaway from a Strange victory on Tuesday, at least in Mr. Trump's eyes, will be that Trump still has clout. The chances that he doesn't claim Strange's victory as his own, at least in private, are infinitesimally small.

And it will indicate to Trump and outside observers that he remains in charge of the Republican base. He put a lot of skin in this primary contest when he could have remained on the sidelines. Instead, he went hard for Strange, and it worked out. The big lesson for Trump will be to keep doing what he's been doing, more or less.

Things get more interesting if Moore wins, which will no doubt be perceived as a real blow to Mr. Trump. If Republicans don't follow the president's lead in deep-red Alabama, where he won by some 30 points, what's the reason to think that they won't put aside his wishes elsewhere?

Any Republican thinking about a 2020 primary challenge to Trump will see a Moore win as a good omen; it may be Trump's party, but it could be up for grabs. And Trump's critics in the press he's so obsessed with will see evidence of an increasingly weak and ineffectual president struggling to keep a grip on the GOP.

Another lesson Trump might glean is that he's been trusting the wrong people as of late. Strange was the establishment choice, the normal, follow-the-rules Republican going up against Moore's unabashed right-wing activist. Trump supporters in the conservative press like Sean Hannity and Steve Bannon threw their support behind Moore, and Mr. Trump will have ignored them at his peril.

The big loser from such a realization? Tough to say, but if Strange loses, it's safe to assume Mr. Trump will have some cross words for Mitch McConnell.