The FBI took it upon itself to determine whether the president of the United States is a threat to national security.

No one had ever before thought that this was an appropriate role for the FBI, a subordinate agency in the executive branch, but President Trump isn’t the only one in Washington trampling norms.

The New York Times reported the astonishing news. “Counter-intelligence investigators,” the paper writes, “had to consider whether the president’s own actions constituted a possible threat to national security.” US presidents over the decades have made many foolhardy decisions that have undermined our security; never before have they been deemed a fit subject for an FBI investigation.

The proximate cause was Trump’s firing of FBI Director James Comey, which the agency considered a potential crime and a national security matter, because it might shut down the investigation into Russian election interference in 2016.

Even if they were shocked by the treatment of Comey, top FBI officials should have quickly ascertained that the Russia investigation continued unimpeded — indeed, is still ongoing today.

If the Times reporting is correct, the FBI grew more suspicious of Trump’s conduct based on comments that have been widely misunderstood. Among the bill of particulars:

During the campaign, he urged the Russians to hack Hillary Clinton’s email. Trump clearly meant this line sardonically, though.

The GOP platform, moreover, allegedly was softened toward Russia. Never mind that, as Byron York of the Washington Examiner has shown, this didn’t actually happen.

And in his Lester Holt interview after the Comey firing, the president said “this Russia thing with Trump is a made-up story.” The president added that he knew firing Comey probably extended the probe rather than shortened it.

More legitimately, agents were disturbed by Trump’s continual praise for Vladimir Putin. These comments were blameworthy but not a federal offense.

The Times implies that foreign policy played into the FBI internal debate. “Many involved in the case,” the paper reports, “viewed Russia as the chief threat to American democratic values.” That is entirely defensible and perhaps the correct view (China is the other candidate). But there is no warrant for the FBI letting its foreign policy ideas influence the momentous decision to ­investigate a president.

As part of the executive branch, the FBI should brush up on the powers of the chief executive. The president gets to fire subordinate executive branch officials. He gets to meet with and talk to foreign leaders. He gets to make policy toward foreign nations. Especially important to the current investigation, he gets to say foolish, ill-informed and destructive things.

If the president wants to tilt ­toward Russia — not that Trump really has except in his words — he can. If he wants to butter up China’s dictatorial president during high-stakes trade negotiations, he can. If he wants to announce a precipitous withdrawal from Syria and make it slightly less precipitous in a fog of confusion, he can.

And the FBI should have nothing to say about it.

The Times story is another sign that we have forgotten the role of our respective branches of government. It is Congress that exists to check and investigate the president, not the FBI. Congress can inveigh against his foreign policy and constrain his options. It can build a case for not re-electing him and perhaps impeach him. These are all actions to be undertaken in the open by ­politically accountable players, so the public can make informed judgments about them.

Perhaps the Times report is ­exaggerated, or the FBI has serious evidence of a criminally corrupt quid pro quo between Trump and Moscow of which there is no public indication yet.

Otherwise, the Times story is a damning account of an offense against our political order, and not by Trump.

Twitter: @RichLowry