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 — On Thursday morning, we told you that the December 12 Alabama Senate race was something you should put on your calendar, especially after Democrats’ big wins earlier in the week. But we had no idea what was going to happen just hours later: The Washington Post reported that a woman said Republican nominee Roy Moore initiated a sexual encounter with her when she was 14 and he was 32.

The news could fundamentally transform the political situation in Washington. If Moore loses next month — and we stress the word “if” — then the GOP’s Senate majority goes from 52-48 to 51-49.

Why is that important? For one thing, it makes it much more realistic for Democrats to capture control of the U.S. Senate in 2018. Democrats would need to win Arizona and Nevada (two races where they have at least a 50-50 shot of victory) and hold on to all of their vulnerable seats. That’s a tall order, of course, but it’s much easier than having to put a Texas or Tennessee on the map to win Senate control. Bottom line: If there’s a wave in 2018, Democrats will have an easier shot of having to flip two Senate seats instead of three to win back the Senate.

In addition, a 51-49 GOP majority in the Senate could imperil the Republican tax plan. Senate Republicans are already having to walk a fine line in placating Sens. Rand Paul (who opposes the tax increases in the GOP plan), Bob Corker (who’s worried about increasing the deficit) and Susan Collins (who is against some of the tax cuts for the wealthy). A 52-48 majority means Senate Republicans can afford to lose two GOP votes, assuming all Democrats oppose the legislation. But with a 51-49 majority, Republicans can lose only one GOP senator.

Now you might say that there’s no way (or little way) that a Republican could lose in Alabama, a state Trump won by a whopping 28 points in 2016. But consider:

Before yesterday, Moore’s lead was just in the high single digits or low double digits, according to the polls. That isn’t a bulletproof lead;

Moore has been a controversial figure in Alabama for more than a decade;

Democratic opponent Doug Jones has owned the TV airwaves for an entire month, with ads like this: “I can work with Republicans better than Roy Moore can work with anyone”;

And the race is a one-on-one special election that takes place two weeks before Christmas, so it will be a low-turnout affair. There is no other race on the ballot.

This isn’t to say that it’s a slam dunk that Moore loses after yesterday. But we’re not sure enough people realize how dangerous the political situation is for the GOP.

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Could Alabama Republicans pull what New Jersey Democrats did in 2002?

And given the situation — as well as the facts that ballots have already been printed and that state law mandates that a candidate must withdraw 76 days before the election to be removed from the ballot (and we’re a month out now) — NBC’s Steve Kornacki reminds us of what happened in New Jersey in 2002:

“In 2002, New Jersey state law set a deadline of 51 days pre-election for a party to switch candidates, but incumbent Bob Torricelli, fresh off his reprimand by the Senate ethics committee and faced with collapsing poll numbers, dropped out 34 days before the vote. Democrats went to court to ask for a ballot switch anyway, and the state Supreme Court – regarded as one of the most liberal in the country – gave it to them. Republicans howled in protest and took it to the U.S. Supreme Court, which refused to hear the case. Frank Lautenberg then subbed in for Torricelli, and won the election easily.”

The lesson: A party that controls the local political power and the courts can change the rules.

But right now, it doesn’t seem like Moore is going to withdraw from the contest. "Judge Roy Moore has endured the most outlandish attacks on any candidate in the modern political arena, but this story in today’s Washington Post alleging sexual impropriety takes the cake," Moore campaign chair Bill Armistead said in statement. "National liberal organizations know their chosen candidate Doug Jones is in a death spiral, and this is their last ditch Hail Mary."

Jonah Goldberg: Saving Moore isn’t worth it for Republicans

Meanwhile, conservative writer Jonah Goldberg criticizes the Republicans who are defending Moore. “… I cannot grasp why so many people think it’s a good idea to stand by a man who, if elected, will serve as a negative campaign ad made flesh. I get the argument that it’s a ‘binary choice!’ But it’s a binary choice now, because a bunch of people who want to see the GOP burn down made it one.”

Goldberg adds, “In the long run, a Senator Moore would cost Republicans more seats than the one he might give them. He’d be an albatross for every elected Republican, including President Trump, who will be asked to take a side on every scene in the clown show Moore would bring to Washington. And every conservative who ever denounces a Democrat for immoral behavior or insane views will be asked, ‘Oh yeah, why did you support Roy Moore then?’ Saving Steve Bannon’s reputation as the leader of some (doomed) movement certainly isn’t worth it, not for the cost to the GOP not to mention your own souls.”

Want to understand why it appeared the GOP was racing for the Moore exits? Goldberg captures that.

Senate Republicans unveil their tax plan

Speaking of the GOP tax effort, Senate Republicans unveiled their plan yesterday, per NBC’s Alex Seitz-Wald, Benjy Sarlin and Leigh Ann Caldwell. “The Senate plan differs in some key ways from the version introduced by the House last week... The Senate bill maintains the current seven individual income-tax brackets, unlike the House's four, and lowers the top rate to 38.5 percent, while the House bill maintains it at 39.6 percent but applies it to higher income.”

“And unlike the House version, the Senate bill would fully repeal the deduction for state and local taxes, which has become a sticking point with GOP congressmen from high-tax states like New York and New Jersey, which tend to lean Democratic, whose constituents depend on that deduction.”

“The House included a deduction for the first $10,000 in property taxes to appease those members, but the Senate, which has fewer Republicans from such states, has no such provision in its bill. In the end, the House may be forced to swallow whatever the Senate passes and it's unclear if dissenting lawmakers from high-tax states have enough numbers to prevent it.”

Was Michael Flynn discussing returning a Turkish opponent living on U.S. soil during the transition?

NBCs Carol Lee and Julia Ainsley: “Federal investigators are examining whether former national security adviser Michael Flynn met with senior Turkish officials just weeks before President Donald Trump’s inauguration about a potential quid-pro-quo in which Flynn would be paid to secretly carry out directives from Ankara while in the White House, according to multiple people familiar with the investigation.”

More: “Investigators for Special Counsel Robert Mueller’s probe into Russia’s attack on the U.S. presidential election recently questioned witnesses about the alleged December 2016 meeting between Flynn and senior Turkish officials, two people knowledgeable with the interviews said. The questions were part of a line of inquiry regarding Flynn’s lobbying efforts on behalf of Turkey.”

“Four people familiar with the investigation said Mueller is looking into whether Flynn discussed in the late December meeting orchestrating the return to Turkey of a chief rival of Turkish President Recep Erdogan who is living in the U.S. Additionally, three people familiar with the investigation said, investigators are examining whether Flynn and other participants discussed a way to free a Turkish-Iranian gold trader, Reza Zarrab, who is jailed in the U.S. Zarrab is facing federal charges that he helped Iran skirt U.S. sanctions.”