As the G.O.P.’s failure to coalesce around a health-care bill demonstrated, the Republican Party under Donald Trump is fractured, perhaps fatally. What began as a unifying goal quickly devolved into a series of proxy wars that pitted moderate Republicans against the party’s far-right wing, including members of the House Freedom Caucus. In the end, with the help of “regular order” purist John McCain, two moderate senators—Susan Collins of Maine and Lisa Murkowski of Alaska—sunk the Senate’s final “skinny repeal” effort, in part because they opposed a provision that made Planned Parenthood ineligible to receive Medicaid reimbursements. Although Republican efforts to rewrite the tax code are not quite as polarizing as stripping health care from millions of Americans, a similar dynamic is now playing out in the Senate, threatening the tax-reform effort on which the G.O.P. has staked its 2018 electoral prospects.

The House managed to pass its tax bill earlier this month, but according to several reports, Senate Republicans are performing last-minute triage on their version of the bill to try to stave off last-minute defections—a pretty crucial goal considering they can only afford to lose two votes to pass the bill with a simple majority. In one ring are Senators Ron Johnson and Steve Daines, who are insisting on more generous treatment of pass-through businesses such as increasing the individual tax deduction for owners of such businesses from the current proposed rate of 17.4 percent. That would be all well and good—ignoring the fact that it would further tilt the benefits of the bill toward the wealthy—if not for the fact that doing so would cost even more money, forcing tax writers to come up with yet more savings to ensure the bill doesn’t cost more than $1.5 trillion over the next decade. Deficit-hawk Senators Bob Corker and Jeff Flake have already said they won’t take kindly to any tweaks that blow out the deficit, leaving Mitch McConnell and Co. with a “Catch-22 seesaw” given that Johnson and Daines’ demands would potentially cost hundreds of billions of dollars.

Meanwhile, Corker, Flake, and Senator James Lankford are now saying that they want to insert some kind of stopgap that would decrease the tax cuts if they don’t unleash the massive economic growth that Trump, Steve Mnuchin, and Gary Cohn have promised. “What if the growth estimates don’t hit the 0.4 percent? What happens?” Lankford said Monday, referring to a Wall Street Journal op-ed predicting an annual G.D.P. boost. “Every economist is guessing.” According to Corker, some sort of the mechanism is being designed for a “trigger or a backstop that in the event the revenues are not there, there’s a way to recoup them so you’re in a situation where you’re not creating deficits should the projections that have been laid out not be real.” (As an aside, please just take a moment to appreciate the words “should the projections that have been laid out not be real.”) Such measures would potentially lose other lawmakers, like Senator Chuck Grassley, who argued that “the sooner [a deficit trigger hits] in the tax cuts, the less benefit the tax cut is.” Meanwhile, McCain is irked about the fact that, at this point, he has no idea what is in the bill, telling reporters, “I have to see what the bill says. It changes every day.”

There is, however, at least one key difference between this vote and the vote to repeal Obamacare. As Senate Whip John Cornyn said, “every single member of the Republican conference is working to get to yes.” (Lindsey Graham was more blunt: “I think all of us realize that if we fail on taxes, that's the end of the Republican Party's governing majority in 2018,” he said on Fox News radio Friday, adding that Trump could find himself impeached.) While the G.O.P. tax plan is opposed by the majority of lawmakers’ constituents, they seem convinced that failing to pass a bill would be worse than passing one that few people like. That perverse, do-or-die mentality could be enough to quash their more granular squabbles—it has already converted Senator Rand Paul, who announced on Monday that he would settle for a compromise. But even if the Senate manages to unite its warring factions, there will still be major obstacles to overcome: House leaders are demanding a conference committee to work out the differences between the two bills, saying they don’t have the votes for the Senate’s version, and that when it comes to the upper chamber’s bill, they will not “swallow it whole.”