Police this week arrested an alleged arsonist who started a fire outside the National Archives building in Washington, claiming that voices told him to “burn buildings down”. The archives display a four-page handwritten document to countless tourists and schoolchildren: the US constitution.

While the physical object remains fragile but secure, the political framework it represents is facing one of the severest threats in its 232-year history. The arsonist is Donald Trump and he is getting ever closer with his tiki torch.

On Wednesday, the House judiciary committee voted to hold the president’s attorney general, William Barr, in contempt of Congress. It was a seminal moment in Democrats’ legal battle with the White House over access to the special counsel Robert Mueller’s report on how Russia helped Trump win the 2016 election.

In response, Trump for the first time invoked the principle of executive privilege, claiming the right to block members of Congress from reading the full Mueller report. The committee chairman, Jerry Nadler of New York, declared the action a clear sign of the president’s “blanket defiance” of Congress’s constitutional right to conduct oversight.

“We are now in a constitutional crisis,” Nadler told reporters after the hearing. “Now is the time of testing whether we can keep our republic, or whether this republic is destined to change into a different, more tyrannical form of government. We must resist this.”

It did not take long for the hashtag #ConstitutionalCrisis to trend on Twitter.

Hyperbole? Republicans and their allies naturally think so. The justice department spokeswoman Kerri Kupec accused Nadler and co of “inappropriate political theatrics”. Geraldo Rivera, a broadcaster on Fox News, tweeted more bluntly: “Anti- @realDonaldTrump pundits & critics have been using the phrase #ConstitutionalCrisis so often over the last 2+ years-It is now seen as alarmist bullshit.”

The House judiciary committee chairman, Jerry Nadler, presiding over Tuesday’s vote. Photograph: Chip Somodevilla/Getty Images

It is true that, from the day he won election, there has been hysteria about the 45th president, or what his supporters like to call “Trump derangement syndrome”. So far he has not jailed journalists, declared martial law or invaded Iraq; institutions have bent rather than broken; the sky has not fallen in. Conventional wisdom has been that, while displaying the authoritarian tendencies of a dictator, Trump lacks the guile to pull it off. Two centuries’ worth of checks and balances should, in theory, be enough to contain him.

Yet this time is different. Alarm bells not heard before are ringing. Not because Trump has got worse – he doesn’t – but because events have forced the matter to a head. Democrats won a majority in the House of Representatives in last November’s midterm elections, obliging them to wheel out a “subpoena cannon” and end the Trump honeymoon in Washington. Then Mueller produced his long-awaited report, chronicling 10 incidents in which Trump may have attempted to obstruct justice but stopping short of indictment, an unsatisfactory conclusion that made all-out political war inevitable.

Dismayingly, Barr has behaved like a political stooge, the sort of apologist one would expect in a slow-moving coup. Now Trump’s assertion of executive privilege – a move normally designed to protect the confidentiality of the Oval Office decision-making process – to hide part of the report and its underlying evidence seems baseless, intended only to trigger a long court battle and run down the clock to election day in November 2020.

Laurence Tribe, a constitutional law professor at Harvard Law School, says: “This is more than minor fireworks. It’s a fundamental challenge to the structure of checks and balances. In particular, the president’s wholesale, blunderbuss assertion of executive privilege over the entirety of the Mueller report is legally groundless to the point of being preposterous.

What we are seeing is a systemic attack on checks and balances Kurt Bardella

“The redacted portions of the report and the underlying masses of evidence – the only items not already in the public domain – include vast amounts of material that cannot conceivably be described as subject to any of the several forms of executive privilege as that privilege was defined in the Nixon tapes case ... This reckless invocation of executive privilege gives the whole concept a stench of coverup, a sad fate for an important principle with a number of entirely valid applications.”

Trump is doing what he always does: defying long-established norms through brute force of will. It was impossible to insult a war veteran like John McCain and get away with it, yet he did. It was impossible to be caught on film bragging about groping women and survive, and yet he did. It was impossible for a president to behave like an absolute monarch and ignore Congress as a co-equal branch of government, and yet he will.

So the White House rejects all congressional efforts to investigate Trump’s business dealings or tax returns as well as the West Wing’s security clearance procedure. It defies a subpoena for the former counsel Don McGahn to turn over documents. Trump says he does not want Mueller to testify on Capitol Hill. All this in the context of a record 58 days without a White House press briefing, a president who constantly attacks the FBI and other institutions and a cable news channel, Fox, that feeds millions of people news from a parallel universe.

Donald Trump seems content to merely stall until the 2020 election. Photograph: Brendan Smialowski/AFP/Getty Images

Writing in the New York Times on Tuesday, the columnist Thomas Friedman noted how, like the spoon-bender Uri Geller, this president bends people to do his bidding. “What worries me most right now is that if Trump gets a second term he’ll also bend all the key institutions that govern us,” Friedman wrote. “Already he’s softening the steel in many of them so they can be bent more easily.”

Trump used to joke about changing the constitution so he could be president for life. The other day he claimed that two years of his presidency had been “stolen” by the drama over Russian collusion and seemed to endorse the idea of a two-year extension as compensation. Perhaps he was still joking. The House speaker, Nancy Pelosi, meanwhile, has warned that if the president narrowly loses next year’s election, he will not voluntarily give up power. It seems a safe enough prediction.

The constitution is well-protected but not fireproof. Kurt Bardella, the former spokesperson and senior adviser for the House oversight and government reform committee, says: “What we are seeing from the Trump administration is a systemic attack on checks and balances that threatens the constitutional balance that our founding fathers established.

“Without a co-equal branch of government to hold the executive branch accountable, we are less of a democracy and more of a dictatorship. This is the most important fight of our republic’s life and if we lose it, we lose the entire idea of American democracy.”