First Read is your briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter.

WASHINGTON — We have no idea who is going to win tonight’s special Senate election in Alabama between Republican Roy Moore and Democrat Doug Jones. (After all, three different polls released Monday showed three very different results.) But we are much more confident about the big stakes involved for the president, his agenda, the Republican Party and the 2018 midterms.

Control of the Senate: A Jones win would reduce the Republican Senate majority from 52-48 to 51-49, giving Democrats a much more realistic chance at retaking the Senate in 2018.

Trump’s legislative and judicial-appointment agenda: One fewer Republican vote in the Senate means less margin for error for Trump’s agenda in that chamber. And while this might not affect tax reform – if he wins, Jones probably wouldn’t be seated for at least a couple of weeks – it could disrupt all other item’s on Trump’s to-do list for 2018.

President Trump: His first candidate in this race (Luther Strange) already lost. And it’s possible that after inserting himself into this contest (the Pensacola rally, the robocall) when other Republicans stepped away from Roy Moore, Trump could go 0-2 in Alabama.

Steve Bannon: If Roy Moore pulls off the victory, you can expect Bannon — the former White House chief strategist — to have even more influence in upcoming Senate primaries in Arizona, Nevada and possibly elsewhere. But if Moore loses, expect Senate Majority Leader Mitch McConnell and his allies to pounce on Bannon, arguing that Bannon’s support for Moore was a key reason why the GOP lost this Senate seat.

The perception of the Republican Party: You could contend that a Moore victory would be the worst of all outcomes for the GOP. As National Review writer David French said on “MTP Daily” yesterday, “I think it’s devastating for the GOP [if Moore wins]. The best way to guarantee that you’re going to get more Roy Moores and more cranks running for GOP offices is to … elect Roy Moore. This guy is going to be a walking campaign commercial for the Democrats – he’s going to make Todd Akin look like a scholar and a gentleman.”

Washington’s sexual harassment story: Finally, a Moore win would only turbocharge the sexual-harassment story that’s rocking Washington and American politics. As Sen. Tim Scott, R-S.C., said on “Meet the Press” last Sunday, there would be an IMMEDIATE ethics investigation into Moore. “We'll have a greater opportunity for us to look into all the issues, the allegations, and perhaps even talk with some of the folks who are witnesses. That will give us a clear picture.”

So a lot is riding on tonight’s outcome — if Moore wins or loses. And consider this: A lot more Republicans are hoping Moore loses tonight. And that’s a significant break with President Trump. The day after a Doug Jones win would be a big story. But the day after a Roy Moore victory would be a REALLY BIG story.

Polling places in Alabama close at 8:00 pm ET.

Doug Jones’ five advantages vs. Roy Moore’s one big advantage

As we wrote last week, Jones enjoys five structural advantages over Moore:

1. Money: Per the last fundraising report, Jones had outraised Moore by nearly a 6-to-1 margin, $10.2 million to $1.8 million.

2. Advertising: Over the airwaves, Jones has outspent Moore by a 6-to-1 margin, according data from Advertising Analytics, with TV ads decrying a broken Washington ("I can work with Republicans better than Roy Moore can work with anyone," he says in one) and a broken health-care system (“Health care is broken. You know it — and I know it," he says in another).

3. Enthusiasm: Whether in purple Virginia or in red Georgia, Kansas and South Carolina, Democratic candidates have overperformed in 2017. And you can probably expect that trend to continue even in Alabama.

4. A low-turnout special election: Don’t forget that this is a special election taking place two weeks before Christmas, with no other major contest on the ballot. And a low-turnout affair benefits Jones — his path to victory is that Republican voters simply stay at home.

5. A historically bad opponent: Finally, and most importantly, Jones is facing off against a historically bad opponent.

But Moore has one structural edge that neutralizes Jones’ five advantages: Alabama is one of the most Republican-leaning states in the country. Donald Trump, after all, won it by 28 points in 2016, 62 percent to 34 percent. And so Moore could underperform Trump by a whopping 25 points and still narrowly win this contest.

Turnout in Alabama could be higher than was expected earlier

The dispatch from NBC’s Mike Memoli and Vaughn Hillyard: “New data provided to NBC News puts the universe of registered voters at 3,326,812 ahead of a hotly contested special Senate election on Tuesday in Alabama. Secretary of State John Merrill, a Republican, has already increased his turnout prediction from 18 to 20 percent last month now to upwards of 25 percent, reflecting the degree to which interest in the contest between Republican Roy Moore and Democrat Doug Jones locally is catching up with national attention.”

“But as local election officials prepared for polls to open Tuesday there were indications that turnout could be even higher still, approaching levels in some cases typically seen in midterm general elections. And neither party is willing to suggest what that says about the final outcome.”

Moore attacks accusers in final campaign event

“On the eve of this state's special Senate election, Republican Roy Moore returned from a six-day campaign hiatus on Monday to attack the women who have accused him of sexual misconduct,” NBC’s Jonathan Allen writes. “Speaking in a barn-style event center here in ‘The Wiregrass,’ the rural and heavily conservative southern tier of the state, Moore portrayed the multiple women as attention-seekers who, he said, ‘had not come forward’ for 40 years but ‘waited until 30 days before this general election to come forward.’”

“And he asked voters to judge his record against their claims. ‘If you don't believe in my character,’ he said, ‘don't vote for me.’”

Jones camp is feeling optimistic

From NBC’s David Ingram and Vaughn Hillyard: “On Monday night, a senior campaign official for the Doug Jones campaign said the Democrat feels good about the candidate’s chances on Tuesday, contending that internal tracking numbers have given it reason to believe its turnout figures will prompt a victory over Roy Moore.”

“The official named Jefferson County, specifically, as the bellwether for the campaign. It’s where Jones, a native of the area, has spent a significant time focusing his efforts, and the campaign hopes a high turnout in the state’s most populous county will reflect strong turnout figures in other key counties.”

Tapping members of Congress to serve in your cabinet creates vacancy headaches

NBC’s Alex Seitz-Wald mused yesterday that, as a political reporter, he’s spent more time this year in Alabama, Georgia, Montana and Kansas — than he did in battleground states like Ohio and Florida.

The reason: All of the major special elections of 2017 — in Kansas, Georgia, Montana, South Carolina and now Alabama — were triggered because Trump tapped sitting senators and members of Congress to serve in his cabinet.

This also was an issue for Barack Obama back in 2009-2010, when vacancies in Illinois (Obama’s Senate seat), Delaware (Biden’s) and Colorado (Ken Salazar) caused major headaches for Democrats.

Maybe a lesson for future presidents-elect: Pick sitting governors and ex-politicians — rather than members of Congress — to serve in your cabinet.