Senate Republicans are in such a rush to pass a tax overhaul in the next few days that they voted to start debate on a bill that could still undergo a bevy of last-minute changes they haven’t seen in writing — changes that could dramatically affect the US economy over the next decade.

But most Republicans aren’t letting some last-minute deal cutting that could mean billions of dollars in tax increases, tax cuts, or federal spending cuts get in the way of moving the bill along.

Even Sen. Bob Corker (R-TN), who’s one of the senators most skeptical of the bill and is pushing for the major addition of automatic tax hikes if the federal deficit grows too quickly, voted to start debate on the bill. He had told reporters earlier that he couldn’t describe the changes “until we get it in writing.” Corker later told reporters after the vote that they could ignore anything they had heard about the deal because it is “still evolving.”

All that happened on Wednesday. Senate Republicans could pass the bill by the end of the week.

The backbone of the Senate bill isn’t changing yet: The corporate tax rate would be slashed from 35 percent to 20 percent, and the individual tax code would be overhauled in a way that sends most of the benefits to the wealthiest Americans. Obamacare’s individual mandate would be repealed, leading to an estimated 13 million fewer Americans having health insurance.

But these last-minute changes might be necessary to bring 50 of the 52 Senate Republicans behind the bill. They would also have far-reaching consequences for years and could determine which Americans reap hundreds of billions of dollars in benefits. Just one alteration under discussion, expanding the tax cuts for “pass-through” firms, would cost $50 billion by some early estimates.

Now is not the time for such details, though, if you ask the senators themselves. Sen. John Kennedy (R-LA) likely spoke for many of his colleagues as he answered reporters’ questions in a Capitol hallway.

“I’m not gonna draw lines. I’m not gonna let the perfect be the enemy of the good. I’m not gonna answer hypotheticals,” he said.

Just the day before, he had indicated he’d never vote for Corker’s “trigger” provision, joking he’d have to be drunk.

But now?

“I may have to get drunk to vote for the bill,” Kennedy said. “But I’m not gonna let the perfect be the enemy of the good, and I’m not gonna draw lines in the dirt.”

What we don’t know about the Senate tax bill right now

Everything is open for negotiation as Senate Republicans search for their elusive 50th vote. President Trump came to the Capitol on Tuesday and seemed willing to agree to about anything, even stabilizing Obamacare, to grease the wheels for a big corporate tax cut.

Significant changes are under discussion to assuage senators worried about pass-through businesses (Johnson and Steve Daines of Montana), the federal deficit (Corker and Jeff Flake of Arizona), and the child tax credit (Marco Rubio of Florida and Mike Lee of Utah), as well as Susan Collins, the Senate’s most moderate Republican, who wants to change the treatment of state and local taxes and to stabilize Obamacare.

Those changes could redistribute hundreds of billions of dollars in the US economy. One outside tax expert guesstimated that Johnson’s desired change for pass-through businesses would cost something like $55 billion over 10 years.

These are the major issues still up for debate as the Senate speeds toward a vote on its tax plan:

The Corker trigger. Corker wants to set up automatic tax hikes if the federal deficit grows too quickly under the GOP tax plan. We don’t know what the threshold would be to initiate those tax increases or which taxes would be increased. Some senators would rather trigger automatic spending cuts instead of tax hikes under this plan.

The pass-through issue. Johnson and Daines have pushed to bump up the share of business income that owners of these pass-through businesses can deduct and exempt from taxation, from 17.5 percent to 20 percent. (By late Wednesday, Johnson was proposing even bigger tax cuts.)

The state and local tax deduction. The Senate’s bill would currently completely eliminate this deduction, to the tune of $1 trillion over 10 years. But Collins wants to allow people to continue deducting up to $10,000 in property taxes, which Trump is said to have agreed to.

The Rubio-Lee child tax credit. The two senators want to further expand the child tax credit and pay for it by slashing the corporate rate slightly less (to 22 percent instead of 20 percent). That could cost, by one outside estimate, $200 billion or more. Trump is said to oppose this proposal.

That’s hundreds of billions of dollars at stake via details still being ironed out, behind the scenes, on the same day that the Senate voted to start debate on the bill.

This whole thing could once again come down to John McCain

One senator might not be too pleased with such sweeping last-minute changes to the tax plan: John McCain.

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, 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.”

Two weeks ago, he was happy with regular order. But that was before the dealmaking got underway.

McCain already wasn’t getting everything he wanted. The tax bill isn’t going to be bipartisan. He’s already voted down the Bush tax cuts because they disproportionately benefited the wealthy.

Now, with the bill suddenly in flux just days before the Senate is supposed to pass it?

Just talked to @SenJohnMcCain.

Where are you on tax bill?

McCain: "Stay tuned."

Have concerns?

"Stay tuned."

What are those concerns?

"Stay tuned"

So will you be talking on the floo-

"Stay tuned" — Seung Min Kim (@seungminkim) November 29, 2017

Republicans nonetheless got the votes to start debate on the bill. Even skeptics like Corker and Flake agreed to take that crucial procedural step. So did McCain.

But final passage could be another story. How this all shakes out in the next 24 to 48 hours could determine whether — and, more importantly, how — Republicans overhaul the nation’s tax code for the next generation.