Meeting with his Cabinet on Monday, the president of the United States, ever a believer in the axiom “overpromise and underdeliver,” vowed to sign “the largest tax cut in the history of our country.” Just as they told the president that Obamacare repeal was a brilliant, slam-dunk first move for his administration, so too his advisers are telling him that tax reform is another no-brainer. A political masterstroke.They are wrong about that. At least, when it comes to Trump. Since Congress appears unable or unwilling to move on almost anything, any tax cut bill—let alone the biggest and best in the history of the world, as the president envisions—will have to overcome serious challenges. But the real problem for Trump is not another legislative failure, but a victory. To paraphrase the immortal words of Admiral Ackbar, the tax cut bill is a trap. If Trump actually does sign it into law, he might as well be signing his political death warrant. I say this not because tax cuts are a bad idea, or unpopular. Nor because tax relief will be invariably characterized, as it always is by liberals, as a giveaway to the rich. The reason the tax cut bill is a danger to Trump is that it’s the one last thing keeping the bulk of his own party in line behind him.One of the most curious storylines in Season One of The President Trump Show is that so many Washington Republicans, inside and outside Congress, are still on board, publicly at least, with a president they clearly denigrate and despise. His own secretary of state may or may not have called his boss a moron. A respected Republican senator publicly questioned Trump’s competence and stability and said he was moving America to the brink of World War III. A special counsel is aggressively pursuing allegations of corruption and collusion that could go all the way to the Oval Office. Trump’s poll ratings are, to borrow a word, sad. He has repeatedly insulted the Republican Senate leader and his colleagues. Faced with all that, especially after Charlottesville and Puerto Rico and endless Twitter feuds and casual falsehoods, you might think any number of GOPers, who notoriously place a priority on their own reputations and careers, would have jumped ship by now—even calling the president unfit to serve in office. Many of them assuredly think that, but none of them has gone as far as to say so publicly. Not yet. There’s a good reason for this—and it’s not that they are gutless wonders, though some undoubtedly are. Trump still has one crucial final task before he can be thrown to the wolves: He must sign a tax reform bill. After that, the wolves can have their quarry.Think stopping a president allegedly hurtling us toward World War III might just be a little more important than passing another round of tax cuts? Well, then you don’t understand the Republican Party, or at least what’s left of it. Today’s GOP is a soulless, brainless beast, a zombie from the Reagan years, when it had its last fresh idea. Since then, the party faithful have defined their priorities around whatever their current leader, interest groups and sympathetic news outlets say they are. A few years ago, for example, it was considered unpatriotic, even traitorous, to suggest the Iraq War was a mistake. In 2016, that very charge won Trump the South Carolina primary. Free trade, fiscal restraint, nation-building—all GOP standards that were cavalierly thrown out of a golden Trump Tower window with barely a word of dissension. Except, that is, for tax cuts. This, the defining issue of the Reagan revolution, is the last unifying idea the Republican Party has left. An idea that even Trump—in all his talk about bringing a wrecking ball to the nation’s capital and his own party—has been loath to dispatch. Tax reform long has been the goal of policy leaders like Speaker Paul Ryan. In one poll commissioned by Ryan allies, nearly half of Republicans say it’s their top priority, and three-quarters call it a major one.I’m not an opponent of tax cuts, and I tend to believe that tax relief can help any number of Americans. But it’s not lost on anyone that among the beneficiaries are large corporations and wealthy Americans—many of whom have hired expensive lobbyists to get this bill passed. Once signed into law, the Republican tax cut plan will give billions of dollars back to nearly every member and supporter of the GOP establishment. To D.C. Republicans, that’s well worth enduring a crazy tweet every now and again, or mouthing support for a wall that will never be built, or nodding agreement about a trade war that will never come to pass, or even standing witness to scary games of one-upmanship with crazy dictators. That’s why every corporate special interest in Washington, every lobbyist and nearly every consultant is holding their fire, and their breath, in the era of Trump. They want to get their piece of the tax cut pie, and getting in fights with the president would only be a harmful distraction.But once Trump signs that bill, he faces his greatest danger: Republicans will finally have an achievement to run on as they seek reelection in 2018. Their donors and supporters will have a prize that eluded them through eight years of Obama, who reversed the Bush-era tax cuts and made them feel like Scrooges who wrecked the global economy. Simply put, they won’t need the president anymore. After that, the investigative team assembled by special counsel Robert Mueller can do its worst. Mueller would actually be doing GOP leaders a favor.With a tax bill behind them, the bigger the better, you will see more Republican members of Congress publicly denouncing the president, and showing far less patience for time-consuming fights with a celebrity sports figure who rubs Trump the wrong way or attacks on the president’s disfavored Republican of the moment. You won’t see leaders watching quietly as Trump encourages divisive primary challenges against incumbents. What you will likely see is real movement toward a well-funded alternative in 2020, should the president even make it that far. And if Mueller does show any evidence of malfeasance on the part of Trump or his team, don’t look for a crowd of Republicans to jump to the president’s defense. Ironically, in an administration filled with ironies, the president’s first chance at enacting a piece of major legislation might also be his last.