With President Trump offering Roy Moore his formal endorsement, the RNC flip-flopping and resuming financial support of his campaign, and Mitch McConnell endorsing Moore as well, it's worth dwelling on the fact that the policy stakes of the Alabama special election are high.

Back before Moore was accused of enjoying sexual predation of teen girls, he was already a controversial figure due to his habit of defying valid court orders, his view that Muslims should be barred from serving in Congress, his view that homosexuality is a "criminal lifestyle," etc. The GOP establishment wanted to nominate someone else for the seat, but when Moore won, they embraced his despite his disregard for the rule of law and the Constitution because — in the immoral words of Rob Portman — "he's going to be for tax reform, I think."

That same calculus applies today.

The House has passed a version of tax reform, and so has the Senate, but while they're broadly similar in spirit, they are different enough that reconciling them is not trivial. And the Senate bill was so hastily drafted that it contains important provisions — most significantly around the corporate alternative minimum tax — that GOP leaders seem to genuinely want to scrap. So they really will have to write another bill and pass it again, and while the odds are very good that they'll succeed, those odds drop a lot if Democrat Doug Jones rather than Moore replaces Luther Strange in the Senate.

With Strange holding the seat, there are 52 GOP senators (plus Vice President Mike Pence), so they can handle the defection of two senators and still pass a bill. If it's Jones instead, they can only handle one defection. And we already know that Bob Corker has decided he objects to the scale of the borrowing involved.

Operating with zero margin for error on such a large and complex bill is dangerous, because it means any single senator can credibly threaten to derail the whole thing unless he gets his pet provisions.

The Wall Street Journal's Kate Davidson and Joshua Jamerson have a great rundown of pet provisions that mysteriously snuck into the Senate bill at the last minute, including:

A favor for mortgage servicers from Mike Rounds

A favor for car dealerships from Rand Paul

A favor for the cruise ship industry from Dan Sullivan

A favor for the oil and gas industry from John Cornyn

If Jones wins the seat, many more legislators will be encouraged to push for similar favors. After all this talk of Republicans reversing the war on coal, for example, surely someone will wonder why coal shouldn't get the same special treatment as oil and gas.

This "Christmas tree" style of legislation, however, poses two problems. One is that scrapping the corporate AMT provision requires the bill to go in the exact opposite direction — they need to inject more spirit of true tax reform with fewer special favors and carve-outs so they don't need that AMT revenue.

The other is that to sell the bill in the House, where party leaders are asking members from rich districts to vote for SALT and mortgage deduction curbs that are bad for their constituents, they really do need to be talking up the team spirit of tax reform. Otherwise, if Dan Sullivan can protect cruise ship operators, why can't New Jersey Republicans protect heavy salt users?

It's telling that Kevin McCarthy, the leader of the California House GOP delegation, immediately demanded the corporate AMT provision be repealed. He's asking his home-state colleagues to swallow a lot for the sake of tax reform. If you turn it into a Christmas tree, that doesn't make sense.

But if Moore loses, it's hard to stop it from turning into a Christmas tree. So at the end of the day, if Republicans are really committed to enacting a large, unpopular corporate tax cut — and they sure seem to be! — then they really need Moore to win the election next week, whether he's a bigot or a sex criminal or whatever else.

This is an abbreviated web version of The Weeds newsletter, a limited-run policy newsletter from Vox’s Matt Yglesias. Sign up to get the full Weeds newsletter in your inbox, plus more charts, tweets, and email-only content.