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This article originally appeared at The Huffington Post. Subscribe here.

The general assumption in Washington has been that only one of two things could cause the Republican leadership to turn on President Trump and begin removal proceedings, either under the 25th Amendment or via articles of impeachment.

One would be a smoking gun so dramatic that Republicans would be compelled to move. That could be some new revelation of direct collusion between Trump personally and the Russian government. Or it could be documented conflicts of interest where Trump explicitly used his office for personal enrichment. Or it might be Trump firing special counsel Robert Mueller, either before or after Mueller issued subpoenas compelling the White House to produce evidence. That, coupled with Trump's ouster of FBI chief James Comey, would compound the president's obstruction of justice.

With each day that Mueller remains on the case, he becomes harder for Trump to fire without incurring fatal damage. Mueller is extremely popular among leading House and Senate Republicans. Republican leaders who are normally Trump apologists have gone out of their way to praise Mueller. If they are saying this in public, they are surely warning the White House privately to keep hands off.

Ousting Mueller would probably require firing Deputy Attorney General Rod Rosenstein as well, setting in motion a Saturday Night Massacre scenario in which Trump had to work his way down the Justice Department hierarchy until he found someone willing to do his bidding.

This would signal that Trump is becoming steadily more unhinged. It could well lead Congress to enact a new law for a special counsel who could not be fired by the executive branch. And it would put immense pressure on Republicans to begin a formal impeachment inquiry. Conversely, if Mueller is not removed, his investigation is likely to unearth details of impeachable offenses that will be hard for Congress to ignore.

No wonder Trump is sputtering mad. He is cornered, and he knows it. (Hey, how about a new social media site for the pathetically enraged: Sputter.com.) The other, complicating development is Trump's standing in the polls. This is moving in a salutary direction, but has not yet reached a tipping point.

Recent polls do show a steady erosion of Trump's approval ratings, now down in the high 30s. The trouble, however, is that national averages are all but meaningless politically.

The House Republican caucus is comprised of individual members who of course are elected district by district. There are 50 or 60 potentially vulnerable incumbents who might be quite inclined to dump Trump in order to save their own skins in the 2018 election.

However these swing seats are more than offset by about 150 Republican incumbents from districts where hardcore Trump support persists. Given these realities, ousting Trump would tear the Republican Party apart.

Thus, a Republican move in the direction of impeachment would require both things-either Mueller's ouster or more dramatic evidence from Mueller's investigation, as well as a deepening collapse in Trump's popular support.

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This does not seem likely in the immediate future. However, never underestimate Trump's capacity for self-destruction.

I don't have a crystal ball. But given all of the items in play, I think the odds are that sometime between now and the end of 2017, a delegation of senior Republicans will pay a call on Trump and suggest that he either resign or face a formal removal process.

Trump does not seem to like his job. He could resign even without that pressure.

If Trump is not forced out, Democrats are likely to take back the House in the 2018 election and begin a formal impeachment process almost as soon as the new Congress convenes in January 2019. But, that's an agonizing 18 months away.

It is positively sickening to contemplate all of the catastrophes that Trump could set off if he has another year and a half in office, much less a full term. One has to hope that the Republican leadership grasps the impending political catastrophe―for their own skins.