It's not unprecedented for senators to part ways with presidents of their own party. But what's happening now among US Republicans is definitely unusual.
Senators Jeff Flake and Bob Corker have openly declared President Donald Trump to be unfit for the office. Senators John McCain and Ben Sasse are close behind.
Scores of other Republican members of Congress share their contempt for Trump but stay quiet either out of fear or a calculation that Republican unity is the path to policy victories like a big tax cut.
But defections of mainstream conservatives like Flake and the others foreshadow a splintering of the Republican Party. Trump wants a party not committed to principles or policies, but only to the Trump brand, however he defines it.
Defections in the Senate aren't new. Democrats-turned-Republicans include Alabama's Richard Shelby in 1994 and South Carolina's Strom Thurmond 30 years earlier. Republicans-turned-Democrats include Vermont's Jim Jeffords in 2001 and Oregon's Wayne Morse in 1955. But these conversions made ideological sense, involving conservatives who found themselves more comfortable with Republicans and liberals who were sympatico with Democrats.
Today's anti-Trump Republicans aren't more or less conservative than the rest of their party. Their objection to Trump concerns his character and competence, and his leadership of a party he defines largely by rage.
Flake has one of the most conservative records in the Senate, but was destined for defeat in a Republican primary next year by a Trump-supporting hatemonger; he said he won't run for re-election.
In a remarkable speech delivered yesterday on the Senate floor, the Arizona lawmaker lamented "the personal attacks, the threats against principles, freedoms and institutions, the flagrant disregard for truth or decency, the reckless provocations, most often for the pettiest and most personal reasons".
He added, "Reckless, outrageous and undignified behaviour has become excused and countenanced as 'telling it like it is' when it is actually just reckless, outrageous and undignified".
Corker, chairman of the Senate Foreign Relations Committee and once a candidate to be Trump's secretary of state, had worked hard to develop a constructive relationship with the President.
But in a CNN interview yesterday, he said that Trump had "great difficulty with the truth". He told ABC News that Trump was "debasing" the US. Like Flake, the Tennessee Republican has decided not to seek re-election next year.
These Republicans generally disagree with Trump's preference for restrictions on trade and immigration, deplore his race-baiting and want more US leadership in solving conflicts around the world. But disagreements on issues aren't the main thing that motivated them.
They believe Trump's positions always are situational, that he lacks core beliefs and that he's determined to remake the Republicans in the Trump brand. There is not a single Republican officeholder to whom he feels any loyalty.
That makes future fissures inevitable, because if Trump is successful he will destroy the mainstream conservative Republican Party.
Some in that mainstream haven't given up on Trump. House Speaker Paul Ryan of Wisconsin, Senate Majority Leader Mitch McConnell of Kentucky and Senator Lindsey Graham of South Carolina are among those sticking with the President because they figure that if they can pass a tax cut and some other legislation, it'll bring more party unity and modify his behaviour.
They should pay attention to this line in Flake's speech: "We have fooled ourselves for long enough that a pivot to governing is right around the corner, a return to civility and stability right behind it. We know better than that."
Flake voted this year for a Republican healthcare bill that was pushed to a vote without hearings and which few supporters even understood; that was indefensible.
He naively supported the successful campaign against appropriations earmarks, which had done a lot to grease the way for constructive compromises. But he is nevertheless one of the more admirable senators I've covered, a principled conservative with an American Conservative Union voting record of more than 93 per cent, one of the highest in the chamber.
The Flake speech may take its historical place alongside the 1950 address by Senator Margaret Chase Smith, her "declaration of conscience" denouncing Senator Joseph McCarthy.
Flake asked his colleagues: "When the next generation asks us, 'Why didn't you do something, why didn't you speak up?' - what are we going to say?"
Were Ryan, McConnell, Graham and Senator Rob Portman listening? Gentlemen, what's your answer?