On Saturday last weekend, Donald Trump alleged in a series of tweets that former President Barack Obama orchestrated a “Nixon/Watergate” plot to tap Trump’s phones at his Trump Tower headquarters last fall in the run-up to the election. Trump concluded that the former president is a “Bad (or sick) guy!”

Last Sunday morning, Trump’s White House called for a congressional investigation.

Trump cited no evidence for his accusation.

Folks, we’ve got a huge problem on our hands. Either:

1 Trump is more nuts than we suspected — a true delusional paranoid. Trump’s outburst was seemingly triggered by commentary March 3 in the alt-right publication Breitbart News, which reported an assertion made the night before by right-wing talk-radio host Mark Levin suggesting that Obama and his administration used “police state” tactics last fall to monitor the Trump team’s dealings with Russian operatives.

If this was the case, we’ve got a president willing to put the prestige and power of his office behind baseless claims emanating from well-known right-wing purveyors of lies. Which means Trump shouldn’t be anywhere near the nuclear codes that could obliterate the planet, or near anything else that could determine the fate of America or the world.

2 The second possibility is that Trump is correct, and the Obama administration did in fact tap his phones. But if this was the case, before the tap could occur, Trump was likely to have committed a very serious crime, possibly treason.

No president can order a wiretap on his own. For federal agents to obtain a wiretap on Trump, the Justice Department would first have had to convince a federal judge that it had gathered sufficient evidence of probable cause to believe Trump had committed a serious crime or was an agent of a foreign power, depending on whether it was a criminal or foreign intelligence wiretap.

In which case we have someone occupying the Oval Office who shouldn’t be making decisions that could endanger America or the world.

3 The third possible explanation for Trump’s rant is that he’s trying to divert public attention from the Jeff Sessions imbroglio and multiple investigations of Trump associates already found to have been in contact with Russian agents during the election, at a time when Russian operatives interfered with the election on Trump’s behalf.

Maybe he’s trying to build a case that the entire Russia story is a plot concocted by the Obama administration — along with the intelligence agencies and the mainstream media — to bring Trump down. That way, he can inoculate himself against more damaging evidence to come.

But if it’s all a big show to divert attention and undermine the credibility of the intelligence agencies and the media, Trump is willing to do anything to keep his job — even if it means further dividing America, undermining trust in our governing institutions, and destroying the fabric of our democracy.

So there you have it. Whatever the reason for Trump’s rant, America is in deep trouble. We have a president who is either a dangerous paranoid who’s making judgments based on right-wing crackpots, or has in all likelihood committed treason, or is willing to sacrifice public trust in our basic institutions to further his selfish goals.

Each of these possible reasons is as terrifying as the other.

But for Democrats to be the only ones sounding the alarm risks turning it into a new normal of partisanship. For Obama himself to respond to Trump’s latest rant would only dignify it.

So the responsibility falls to Republican leaders. They must stand up and call this what it is: dangerous demagoguery.

We are depending on former Republican Presidents George H.W. Bush and George W. Bush, former Republican senators and members of Congress, and current Republican senators and members of Congress, to have the courage and decency to stop this outrage.

We are in a serious crisis of governance, and their voices are critical.

© 2017 Robert Reich

Robert Reich, a former U.S. Secretary of Labor, is professor of public policy at UC Berkeley. His daily blog is at www.facebook.com/RBReich/. To comment, submit your letter to the editor at http://bit.ly/SFChronicleletters.