Once again, John McCain might hold the fate of President Donald Trump and the Republican Party’s biggest legislative priority in the palm of his hands.

Where does the Arizona senator, the maverick who helped sink Obamacare repeal this summer, stand on the GOP tax bill?

He’s not really saying.

McCain says he's still undecided on the tax bill. Concerns? "A lot of things." — Richard Rubin (@RichardRubinDC) November 27, 2017

McCain is likely to be a critical vote. Senate Republicans need 50 of their 52 members to support the tax plan. With a few deficit hawks and moderates like Sen. Susan Collins (R-ME) in doubt, he could end up being the 50th vote in favor or the third vote against.

The senator, who was diagnosed with brain cancer earlier this year, could probably find reason to oppose the plan if he wanted to. But his style throughout the year has been to keep his cards close to his chest. Before dramatically voting down Obamacare repeal earlier this year, McCain only offered to reporters: “Watch the show.”

What does McCain want? He’s been preoccupied with the process that produced the tax bill, demanding “regular order” rather than a rushed and secretive negotiation. He has praised Republican leaders at times for running the tax bill through regular order, but other times blanched at last-minute changes, more of which could be coming. The Senate is currently debating a major alteration that would trigger automatic tax hikes if the federal deficit grows too much in later years.

He also wanted a bipartisan bill, whereas Senate Republicans decided to pass their tax overhaul through budget reconciliation explicitly so they wouldn’t need Democrats to vote for it. No Senate Democrat is expected to vote for the GOP tax plan, as Vox’s Ella Nilsen reported.

McCain also has a history of opposing tax plans that favor the nation’s wealthy at the expense of its middle class. He voted against the George W. Bush tax cuts twice, one of only a few Republicans to oppose them, on those grounds. Most analyses project that the top 1 percent would yield most of the benefits of this new Republican tax plan and many poor and middle-class Americans would actually pay higher taxes than now.

McCain could very well vote to start debate on the tax bill, which is expected to come as soon as Wednesday, much as he did in the health care drama. He doesn’t want to hold up the process. But when the actual vote to pass the plan comes, all eyes will be on him and his thumb once again. Up or down?

The tax bill went through “regular order” but not regular order

McCain obsesses over regular order: the series of committee hearings, expert testimony, and amendments that are supposed to produce a bill. He based his vote against the Obamacare “skinny” repeal bill, which ended the Republican hopes at the time of undoing the law, largely on the fact that the plan was rushed together mere hours before it was put on the Senate floor.

He has been much more positive about the process that produced the tax overhaul — though this bill now also includes a version of “skinny repeal” — explicitly praising it as regular order when the finance committee approved the plan earlier this month.

“I am pleased that the Finance Committee has followed the regular order by holding numerous hearings and spending four days debating the bill and considering amendments in committee,” McCain said in a statement. “As chairman of the Senate Armed Services Committee, I value the process of moving important pieces of legislation through regular order.”

There have been hearings and a formal mark-up of the Senate tax bill in marked contrast to the health care bill, negotiated and drafted entirely both closed doors. But it still might be a stretch to call it true regular order, according to two former Senate aides I consulted.

“Regular order is different to different people. The most basic form of regular order is the School House Rock ‘bill on a hill’ stuff,” a former Democratic aide for the Senate Budget Committee told me. “Committee hearings, mark-up and amendments, going to the floor, amendments on the floor, passing both chambers, resolving differences, signature or veto.”

“Under that bare-bones description we certainly seem to following regular order on the tax bill and probably kosher for McCain’s request,” the former aide added. “Even with all this wheeling and dealing at the moment, it still ‘feels’ like regular order even with the tighter time frame because the bill was developed in committee.”

But in a different sense, the Republican tax bill has been anything but regular. It goes back to leadership’s decision to use budget reconciliation, according to the Democratic aide and Bill Hoagland, a former Republican budget staffer who is now with the Bipartisan Policy Center.

Budget reconciliation allows a bill to advance with only 51 votes instead of the usual 60. That allowed Republicans to move their tax plan without any Democratic support, even though McCain has said he wants a bipartisan bill. Senate classicists contend that the reconciliation process was never meant to support such sweeping domestic policy changes like the Republican tax plan.

“While the budget process regular order has been followed, I fault this process on the grounds that it was used for only one purpose to develop the tax reconciliation bill to avoid having to face a 60-vote threshold in the Senate,” Hoagland told me. “Here I believe the drafters of the Budget Act never envisioned that major public policy legislation that impacted the economy and individuals as much as taxes and health policy does should not face the full, unfettered scrutiny of the Senate.”

So the tax bill is “regular order” only under a contorted definition — and it certainly hasn’t been bipartisan, as McCain wanted. In fact, it’s been run through this explicitly partisan process.

McCain has voted against tax cuts that favor the rich before

But, as Moller said, regular order can mean just about anything you want it to. McCain sounds mostly satisfied, though Republican leaders are weighing last-minute changes to the tax bill to assuage iffy votes. To win over deficit hawks like Sen. Bob Corker (R-TN), the final bill could trigger automatic tax increases on corporations if the deficit is growing too fast. But the details haven’t been ironed out on that major provision mere days before an expected vote.

McCain could balk if the final legislation has been significantly overhauled. He has before.

But on the substance, McCain also has good reason to oppose the Republican tax plan: It directs its biggest benefits to wealthy Americans, while eventually raising taxes on millions of poor and middle-class people.

That was the substantive objection that McCain raised when he voted against the Bush tax cuts in 2001 and 2003.

“I cannot in good conscience support a tax cut in which so many of the benefits go to the most fortunate among us, at the expense of middle-class Americans who most need tax relief,” he said in a Senate floor speech in 2001. He opposed the 2003 renewal of the tax cuts because they were "too tilted to the wealthy."

The centerpiece of the Republican tax plan is a huge corporate tax cut, at a time when most Americans think businesses should be taxed more not less. It also slashes taxes on “pass-through” businesses disproportionately used by rich people (like Trump) to lower their tax burden. The top tax rate for the richest Americans would also be cut, and the estate tax for wealthy inheritances would be rolled back.

And because the budget reconciliation rules — made necessary because Republican leaders wanted to pursue a partisan bill — many of the individual tax cuts would expire in later years, leading to a significant tax hike for millions of people if Congress doesn’t act.

When you factor in the repeal of Obamacare’s individual mandate, which is projected to lead to 13 million fewer Americans having health insurance and therefore less federal spending on Medicaid and tax subsidies for lower-income and middle-class folks, the disparity becomes even sharper.

McCain has been reluctant to talk publicly about the substance of the tax bill, preferring to vaguely allude to his “concerns,” as he told the Wall Street Journal’s Richard Rubin earlier this week. But the bones of the tax plan are tax cuts that would favor the rich and businesses on the backs of the poor and the middle class.

That is a proposal that McCain has voted down before. He could now be the decisive vote against a similar plan over the next few days. But only the distinguished senator from Arizona knows exactly what kind of show he’s planning for this time.