Meanwhile, behind closed doors, William B. Taylor Jr., the U.S. chargé d’affaires to Ukraine, told Congress on Tuesday that Trump had indeed made aid to Ukraine contingent on its government publicly announcing an investigation into former vice president Joe Biden’s son Hunter and the 2016 election. That seems to eviscerate the administration’s claims that there was no quid pro quo or that the demands had some sort of legitimate state purpose.

At this point, it’s clear Trump is going to be impeached. What’s open to question is whether the Senate will remove him. For a long time, that answer had seemed equally obvious: Of course they wouldn’t. There would be a quick trial amid complaints about “lynching” and similar rhetoric, and Trump would stay in office. That’s still the conventional wisdom — but allow me to outline why I think it might be wrong.

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Start with the fact that congressional Republicans don’t much like him. They defend him mostly because they are afraid of his loyalists. It’s hard to know how numerous those voters are, but here’s one reasonable proxy: On Super Tuesday 2016, when the Republican primary race was still hotly contested, Trump failed to garner 50 percent of Republican primary voters in any of the 11 states.

Nonetheless, GOP senators need their support to win. While most of them would like to get rid of Trump, any individual who stands up to him faces a backlash from the Trump loyalists — and while they could probably prevail if they all shifted at once, so far, they haven’t been able to coordinate a collective action.

Perhaps because they understand that coordination problem, Trump loyalists interpret any criticism of the president, however mild, as an attempted countercoup by resentful elites. And hence for three years we’ve watched congressional Republicans treading gingerly between loyalists who demand obeisance, and Trump-skeptical moderates who want independence.

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They’ve been able to stay balanced on that tightrope mostly because Trump’s approval rating has been so stable — 42 percent, plus or minus 4 percentage points, for virtually his entire presidency. But any movement among the voters could upset the Senatorial equilibrium, and that’s not necessarily as unlikely as it currently looks. Humans are fundamentally herd animals, even (perhaps especially) when it comes to our beliefs. A few bellwethers breaking away at the right time can lead the herd in an entirely different direction — and much more rapidly than you might think, as the opponents of same-sex marriage discovered not long ago.

With Trump, such a scenario might start with hearings that expose the public to his administration’s internal decision-making processes, at length, and not filtered through anonymous sources and a left-leaning mainstream media. Embarrassed and outraged, Trump starts lashing out more vehemently and unpredictably — say, by whimsically authorizing Turkey to slaughter a bunch of Kurds who, among other things, happen to be guarding thousands of Islamic State fighters we would like to stay locked up.

As Trump’s ill-conceived outbursts start generating undeniable real-world consequences rather than eliciting fuzzy complaints about “civility” and “norms,” they could cost him his more weakly attached voters. More and more Republicans hear from previously Trump-loyal friends that that’s it, they’re through. That opens up emotional space for them to consider rejecting him, too.

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With Trump’s numbers worsening, it will then become harder to maintain the illusion that Trump is somehow immune to normal political rules. By the time the poll numbers for removal inch up to somewhere in the range of 55 to 58 percent, it becomes clear Trump will almost certainly lose in 2020 — and worse, take the Senate down with him.

Now his support really begins to collapse, particularly among evangelical Christians. A hard core of Trump supporters, perhaps a quarter of the electorate, will stick with him to the bitter end. But he relies on another big chunk who don’t like him yet rely on him for regulatory and judicial protection from the social justice warriors.

Such transactional support might evaporate if Trump alienates so many independents that even his more tepid supporters conclude that come 2021, Democrats will be appointing all the judges. At that point, what’s the benefit of another year of Trump — another year, for his supporters, of making excuses for behavior that most of us wouldn’t accept in a toddler? Another year of letting the toddler run our foreign policy?

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If Trump can’t win another term, then why not take a chance on President Pence? He might be able to save the Senate. At the very least, evangelicals could vote their conscience rather than their self interest — and if that sounds nice, why not allow their senators the same luxury?