As it stands, Republicans are on the verge of accomplishing a goal that’s been a major plank of their platform for more than three decades. Despite how close they’ve come to realizing their tax-reform fever dream, however, on Monday Senator Orrin Hatch described the mood among G.O.P. lawmakers as dour. There’s trepidation, he said, and a heavy sense of resignation that Donald Trump might change his mind on the bill at the last minute, inadvertently backing lawmakers into a corner. “We need to know what the president wants to do to try to coordinate it with him,” he said. “So far, I’m not quite sure where he’s going.”

Hatch was referring to the president’s tweet earlier that day rejecting a G.O.P. proposal to reduce the limits on annual 401(k) contributions, keeping his campaign promise but cutting off an avenue for lawmakers scrambling to offset massive, multi-trillion dollar revenue losses resulting from the tax cuts. Other G.O.P. proposals in the works: phasing in the corporate tax rate over the next 10 years instead of cleanly chopping it to 20 percent; eliminating deductions for out-of-pocket medical expenses; or—horror of horrors—making corporate and personal tax cuts temporary, may elicit similar responses.

Even if the president initially seems amenable to such proposals, there is always the risk that Trump will change his mind depending on what cable news show he’s watching. “Trump engages in fits and starts and then undermines his side’s negotiating positions half the time,” Tim Miller, a former communications director for Jeb Bush’s presidential campaign, told The New York Times. “He doesn’t care about the details of tax reform, or Obamacare repeal and replace, and thus inserting himself into the negotiations has been largely counterproductive.” Republican lawmakers have learned not to rely on Trump’s word; he “can shift on a dime, and he has many unformed policy positions,” G.O.P. Representative Charlie Dent told the Times. “We have to worry about him shifting positions.”

If this all sounds familiar, recall Republicans’ numerous attempts to repeal and replace Obamacare, wherein Trump repeatedly changed positions on his own party’s proposals, celebrating the House’s passage of a repeal bill in the Rose Garden one day, and bashing it behind closed doors the next. (The same phenomenon currently afflicts a bipartisan Obamacare stabilization bill in the Senate, on which Trump has now taken some half-dozen conflicting positions.) As Jon Lieber, a former Mitch McConnell aide, put it, “The Trump calling things ‘mean’ threat is very real right now.”