Donald Trump is feeling good about taxes. In his gonzo press conference last Thursday, he assured Americans that “very historic tax reform” is absolutely on track and is going to be—wait for it!—“big league.” The week before, he told a bunch of airline CEOs that “big league” reform was “way head of schedule” and that his people would be announcing something “phenomenal” in “two or three weeks.” And at his Orlando pep rally this past weekend, he gushed about his idea for a punitive 35 percent border tax on products manufactured overseas. The magic is happening, people. And soon America’s tax code will be the best, most beautiful in the world.

But here’s the thing. What Trump doesn’t know about the legislative process could overflow the pool at Mar-a Lago. And when it comes to tax reform, even minor changes make Congress lose its mind. Weird fault lines appear, and the next thing you know, warring factions have painted their faces blue and vowed to die on the blood-soaked battlefield before allowing this marginal rate to change or that loophole to close.

Such drama has, in fact, already begun over the proposal percolating in the House. At issue: a provision known as the border adjustment tax—let’s call it BAT—which, shrunk to its essence, incentivizes domestic manufacturing by slapping a 20 percent levy on imports, while making U.S. companies’ export-revenues tax deductible.

BAT fans—most notably House Speaker Paul Ryan and Ways and Means Chairman Kevin Brady—pitch the provision as an economically elegant twofer: an America-First measure that discourages companies from moving operations overseas while creating a revenue stream ($1 trillion every decade or so) that allows the overall corporate tax rate to be slashed.

Opponents—most vocally Senators David Perdue and Tom Cotton—argue that a BAT is another grubby government cash grab that will ultimately hurt consumers when, say, Walmart has to jack up the prices of underwear, bananas, and Playstations. In a February 8 letter to colleagues, Perdue, who spent four decades in the business world, charged that the BAT is “regressive, hammers consumers, and shuts down economic growth.”

Thus the battle lines are drawn. And, make no mistake, this will not be some bush-league, penny-ante skirmish. Behind the legislative factions are amassing some of the heaviest hitters in corporate America, ready to spend millions to sway debate on behalf of their team.

Roughly speaking, companies that do a lot of exporting dig the BAT (think: Boeing, Merck, and Dow Chemical) while import-dependent retailers (including Target, Nike, and, yes, Walmart) fear it will destroy their bottom lines. The oil industry isn’t feeling much BAT love either. The Koch brothers want it dead, like, yesterday.

At this point, anti-BATers have an edge. Why? Partly, because the provision is super complicated and almost impossible to explain in terms that don’t sound like something a coven of economists vomited up. Ask BAT fans why the provision won’t, in fact, hurt retailers or consumers, and you’re instantly hip-deep in talk of currency revaluation, purchasing power, and territorial taxation. Last Wednesday, one day after Paul Ryan tried to educate Senate Republicans on the wonders of BAT at their weekly policy lunch, Tom Cotton (who represents Walmart’s home state of Arkansas) snarked on the Senate floor, “Some ideas are so stupid only an intellectual could believe them.”

This is in no way to suggest that the pro-BAT arguments are wrong. They simply don’t push the same buttons as anti-BAT warnings that Congress is poised to screw consumers in order to fund big tax cuts for corporations.

For the past few weeks, in fact, an anti-BAT coalition called Americans for Affordable Products has been busy hawking this exact message. “This is a consumer tax—a means by which House Republicans are paying for other tax deductions,” asserted AAP member Brian Dodge. “It’s not about America First. It’s not a trade-deficit reduction tool. It is a pay-for.”

AAP is lobbying lawmakers and staffers and doing public outreach. Last Wednesday, it dispatched eight CEOs to chat with Trump and Vice President Pence. “We view our job as leading a large education campaign,” said Dodge. “We believe the more that lawmakers understand about this proposal, the less inclined they’ll be to support it.”

Of course, BAT fans are gearing up as well and promise to be equally aggressive. The day after the AAP roll out, the American Made Coalition launched, with an eye toward helping Ryan’s office spread the good word. “It takes time to educate both policy makers and businesses on what’s on the table,” said Brian Reardon, an adviser to the group.

There is no place for subtlety in this war. Part of BAT supporters’ argument is that, without the provision, tax overhaul will implode altogether. Message: Get on board or kiss your once-in-a-lifetime reform opportunity good-bye.

It’s a question of Senate math. To pass with a simple majority (and avoid a filibuster by Democrats), the GOP’s plan must go through under the procedure known as reconciliation. But to qualify for reconciliation, the package--which slashes both corporate and upper-bracket taxes--cannot blow a hole in the long-term budget. Without the $1 trillion in revenues from BAT, say advocates, there’s no way that hole can be plugged.

“This is the only way at these rates and keeping things revenue neutral,” insisted a senior Republican aide. There is no other viable option. Period. End of story.

But anti-BATers are eyeing a different Senate equation. To amass even a simple majority of votes, the BAT can lose only two of the 52 Republican members. (Unless Democrats cross the aisle, of course.) In addition to Cotton’s and Perdue’s open hostility, Senators John Boozman, Mike Rounds, John Cornyn, Tim Scott, and Mike Lee have all expressed reservations. "I have real concerns that this piece of the House blueprint will cause more disruption than necessary,” Lee said. “Will the dollar suddenly shoot up by 20 percent? Will U.S. manufacturers have to redo their international supply chains? These are all open questions.”

With the provision’s Senate prospects iffy, there’s less incentive for House conservatives to support something that smells even faintly like a tax. Both the current chairman of the Freedom Caucus, Mark Meadows, and the former chairman, Jim Jordan, have said they’d like reform done without a BAT.

“My reasoning is very basic,” Jordan told me. “Why in the world would we want to add another revenue stream?” You can debate the impact on exchange rates and purchasing power all day, said Jordan, but that doesn’t address many conservatives’ core objection. “We come at it from fundamental perspective,” he said. “The idea that you’re going to add an entirely new tax is a big problem.”

(BAT fans, for the record, dispute that this is a new tax. It is, they insist, replacing the existing system with an entirely new, far superior one that must be looked at, as Reardon put it, “holistically.”)

The only thing everyone can agree on is that this will be a long, ugly fight. If Trump drops his tariff idea and embraces BAT, it could boost the cause. But even then, he’d need to do major arm-twisting to get Senate skeptics on board (especially with the likes of Walmart and the Kochs twisting the other arm.) Like it or not, this is what the political big leagues are like: slow, messy, and infuriating.

The up side for Trump: He’ll have time to throw a lot more pep rallies on this topic before anything gets decided.