Up to now I have not favored removing President Trump from office. I felt strongly that it would be best for the country that he leave the way he came in, through the ballot box. But last week was a watershed moment for me, and I think for many Americans, including some Republicans.

It was the moment when you had to ask whether we really can survive two more years of Trump as president, whether this man and his demented behavior — which will get only worse as the Mueller investigation concludes — are going to destabilize our country, our markets, our key institutions and, by extension, the world. And therefore his removal from office now has to be on the table.

I believe that the only responsible choice for the Republican Party today is an intervention with the president that makes clear that if there is not a radical change in how he conducts himself — and I think that is unlikely — the party’s leadership will have no choice but to press for his resignation or join calls for his impeachment.

It has to start with Republicans, given both the numbers needed in the Senate and political reality. Removing this president has to be an act of national unity as much as possible — otherwise it will tear the country apart even more. I know that such an action is very difficult for today’s G.O.P., but the time is long past for it to rise to confront this crisis of American leadership.

Trump’s behavior has become so erratic, his lying so persistent, his willingness to fulfill the basic functions of the presidency — like reading briefing books, consulting government experts before making major changes and appointing a competent staff — so absent, his readiness to accommodate Russia and spurn allies so disturbing and his obsession with himself and his ego over all other considerations so consistent, two more years of him in office could pose a real threat to our nation. Vice President Mike Pence could not possibly be worse.

The damage an out-of-control Trump can do goes well beyond our borders. America is the keystone of global stability. Our world is the way it is today — a place that, despite all its problems, still enjoys more peace and prosperity than at any time in history — because America is the way it is (or at least was). And that is a nation that at its best has always stood up for the universal values of freedom and human rights, has always paid extra to stabilize the global system from which we were the biggest beneficiary and has always nurtured and protected alliances with like-minded nations.

Donald Trump has proved time and again that he knows nothing of the history or importance of this America. That was made starkly clear in Secretary of Defense Jim Mattis’s resignation letter.

Trump is in the grip of a mad notion that the entire web of global institutions and alliances built after World War II — which, with all their imperfections, have provided the connective tissues that have created this unprecedented era of peace and prosperity — threatens American sovereignty and prosperity and that we are better off without them.

So Trump gloats at the troubles facing the European Union, urges Britain to exit and leaks that he’d consider quitting NATO. These are institutions that all need to be improved, but not scrapped. If America becomes a predator on all the treaties, multilateral institutions and alliances holding the world together; if America goes from being the world’s anchor of stability to an engine of instability; if America goes from a democracy built on the twin pillars of truth and trust to a country where it is acceptable for the president to attack truth and trust on a daily basis, watch out: Your kids won’t just grow up in a different America. They will grow up in a different world.

The last time America disengaged from the world remotely in this manner was in the 1930s, and you remember what followed: World War II.

You have no idea how quickly institutions like NATO and the E.U. and the World Trade Organization and just basic global norms — like thou shalt not kill and dismember a journalist in your own consulate — can unravel when America goes AWOL or haywire under a shameless isolated president.

But this is not just about the world, it’s about the minimum decorum and stability we expect from our president. If the C.E.O. of any public company in America behaved like Trump has over the past two years — constantly lying, tossing out aides like they were Kleenex, tweeting endlessly like a teenager, ignoring the advice of experts — he or she would have been fired by the board of directors long ago. Should we expect less for our president?

That’s what the financial markets are now asking. For the first two years of the Trump presidency the markets treated his dishonesty and craziness as background noise to all the soaring corporate profits and stocks. But that is no longer the case. Trump has markets worried.

The instability Trump is generating — including his attacks on the chairman of the Federal Reserve — is causing investors to wonder where the economic and geopolitical management will come from as the economy slows down. What if we’re plunged into an economic crisis and we have a president whose first instinct is always to blame others and who’s already purged from his side the most sober adults willing to tell him that his vaunted “gut instincts” have no grounding in economics or in law or in common sense. Mattis was the last one.

We are now left with the B team — all the people who were ready to take the jobs that Trump’s first team either resigned from — because they could not countenance his lying, chaos and ignorance — or were fired from for the same reasons.

I seriously doubt that any of these B-players would have been hired by any other administration. Not only do they not inspire confidence in a crisis, but they are all walking around knowing that Trump would stab every one of them in the back with his Twitter knife, at any moment, if it served him. This makes them even less effective.