By now no one should be surprised when Donald Trump throws out wild accusations and conspiracy theories.

The president launched his national political career on the racist and false “birther” lie that former President Barack Obama is not a real American and was not born in the United States. During the presidential campaign, Trump falsely claimed that “thousands and thousands” of “Arab[s]” in New Jersey cheered on September 11, 2001 as the Twin Towers fell in New York City. After the November 2016 election, Trump claimed, without evidence, that millions of people had voted illegally.

Perhaps many Americans have become accustomed to the president’s trumped-up tales about matters large and small, but even the most jaded citizen should be jolted into high alert after Trump’s latest unsubstantiated accusation.

On Saturday, Trump claimed in a series of early morning tweets, without citing any evidence, that Obama authorized the wiretapping of Trump Tower before the November 2016 election. Reporting suggests that Trump “may have been responding to Breitbart commentary” which itself breathlessly and baselessly speculated that Obama sought court approval ““to monitor Trump and his senior advisers.” In a tweet, Trump commented that Obama’s actions were reminiscent of President Richard Nixon and the Watergate scandal, and called Obama a “Bad (or sick) guy!”

“ Trump appears to be playing a high-stakes game of ‘telephone.’ ”

Trump appears to be playing a high-stakes game of “telephone,” in which one person whispers a message to the next that becomes more and more garbled over time. This is a fun diversion for children, but dangerous and irresponsible in America’s leader.

Americans ought to consider the pros and cons of a president who utters paranoid accusations suggesting criminal wrongdoing by his predecessor without the barest suggestion of evidence. This is not a partisan issue, but a matter of common sense.

Trump, without evidence, accuses Obama of wiretapping him

To their credit, some congressional Republicans get this. Michigan Rep. Justin Amash asserted that Trump’s charges have “serious implications” and the president “should provide more info[rmation] to Congress immediately to assess constitutionality and legality.” Sen. Ben Sasse of Nebraska agreed that “the President made some very serious allegations, and the informed citizens that a republic requires deserve more information.”

Sasse added that “We are in the midst of a civilization-warping crisis of public trust, and the president’s allegations today demand the thorough and dispassionate attention of serious patriots.”

Those are strong words, to say the least, and they are merited. Trump has launched an attack on American institutions — the press, the intelligence community, the courts. As Sasse suggests, this is all having a corrosive effect.

We’ve seen enough. It’s past time to take action to defend and repair America’s federal institutions, its constitutional democracy, and the bedrock belief that public discourse should be based on facts.

“ Americans must know — as soon as possible — what, if anything, Trump is talking about. ”

What can we do? Republicans like Sasse and Amash, who recognize that Trump’s serious charges require serious evidence, should join their Democratic colleagues to initiate a nonpartisan investigation into Trump’s charges. Americans must know — as soon as possible — what, if anything, Trump is talking about. White House press secretary Sean Spicer on Sunday called for an investigation into Trump’s claims. By all means, let’s find out what’s going on here, and call out what is almost certainly Trump’s reckless bluff.

The president needs to put up or shut up. If Trump can identify evidence to show that Obama personally ordered wiretapping of Trump’s phones, that would be a serious matter. If, on the other hand, Trump has no evidence to back up his incendiary allegations, that would also be a serious matter, and there must be consequences.

America cannot have a president who believes it is appropriate to suggest his predecessor engaged in criminal wrongdoing without presenting evidence to support such claims. A U.S. president wields immense power, including the authority to order nuclear strikes. What happens if Trump reads a Breitbart article which leads him to conclude incorrectly that military action is necessary?

If an independent investigation concludes that Trump’s charges were made recklessly and without evidence, then it would be appropriate for the U.S. government to invoke the Constitution’s 25th Amendment, which allows the vice president and a majority of the cabinet (or some other body designated by Congress) to declare in writing that the president is “unable to discharge the powers and duties of his office.”

The vice president would then become acting president. If the president objected, Congress would decide: a two-thirds majority would be required to confirm the president’s inability to serve.

This is an extreme measure, to say the least, and it should not be taken lightly. But Trump is an extreme president. An independent investigation is necessary to get to the bottom of Trump’s charge against Obama. If it turns out Trump’s dawn tweetstorm was made without any evidence, if Trump falsely charged Obama with violating criminal law, then extreme action is warranted.

As has been noted in a different context, the Constitution is not a suicide pact. It is past time for Americans to begin the hard work of resolving the ”civilization-warping crisis of public trust” that Trump is leveraging and which is weakening our nation.

Chris Edelson is an assistant professor of government in American University’s School of Public Affairs. His latest book, Power Without Constraint: The Post 9/11 Presidency and National Security, was published in May 2016 by the University of Wisconsin Press.