Donald Trump says he could shoot someone on New York City’s Fifth Avenue and keep his Republican base. He may be right.

But if special counsel Robert Mueller flips over a single bit of hard evidence that Trump conspired with the Russians to win the 2016 election, expect Republican leaders to throw him to the dogs like day-old McDonald’s. And they’ll do it because they want to, not just because they’ll have to — they’ll finally have the excuse they need with Trump’s base to end this excruciating political nightmare.

Campaign finance violations, however salacious — could they be anything other with this president? — or even obstruction of justice, which forced President Richard Nixon to resign, might be politically survivable for a resilient Trump. But Russia would be different. If it can be proved that Trump’s campaign was in communication with Russian President Vladimir Putin’s cyber weapons operations in 2016, or worse, that U.S. policy has been altered as a result of underhanded dealings with Putin, Trump will be abandoned. Conspiring with the enemy is 100 percent fatal, last I checked.

The conversation has already begun in influential GOP circles: Who else is out there for 2020? Would Trump ever resign? Would he forgo a re-election run? The words are being whispered in earnest for the first time — even as the Republican National Committee announces plans to fully entwine itself with the 2020 Trump campaign. All ears are to the ground; all eyes are on Mueller.

If a breakup does comes to pass, it should come quickly and as no surprise: GOP loyalty to Trump has always been fallacious. Republican leaders never liked Trump, even the ones who defend him on TV. He was choice number 17 in a 17-candidate primary field, for most of them, a candidacy thrust upon the Republican Party by exasperated voters it could neither control nor understand. Fear of that constituency — the need for it electorally — is what’s kept Republican leaders in lock step, never loyalty to a president they don’t respect.

Florida Sen. Marco Rubio presaged in a recent interview with “Meet the Press” what others will surely say should the Mueller probe strike bone: “If someone has violated the law, the application of the law should be applied to them like it would to any other citizen in this country, and obviously if you’re in a position of great authority like the presidency that would be the case,” Rubio said. “It will not be a political decision,” he emphasized, “it’ll be the fact that we are a nation of laws and no one in this country no matter who you are is above it.”

In other words, Trump loyalists, our hands will be tied.

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Even ostensible allies like Sen. Lindsey Graham of South Carolina are beginning to distance themselves from the embattled president. He slammed Trump this week for precipitously announcing a troop withdrawal from Syria, the ardent wish of Vladimir Putin it must be noted. Others in the House and Senate have as well with seeming and sudden impunity, an unmistakable sign that the worm has started to turn.

It’s impossible to know what Mueller has up his sleeve. But the cards he has laid down so far have been enough to start the conversation. It will be a difficult one to stifle.

William F. B. O’Reilly is a Republican consultant.