The week started out as a quiet one. The President had returned to Washington from his golf club in New Jersey and there were no public events on his schedule. The U.S. House of Representatives was on a week-long break after passing a health-care bill, the President's first significant legislative victory.

But by late Tuesday afternoon, Donald Trump had unleashed a maelstrom.

He summarily fired James Comey, the director of the Federal Bureau of Investigation, just as the agency was probing possible collusion between Mr. Trump's election campaign and Russia.

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The dismissal toppled a long-standing tradition of the president not doing anything construed as interfering in the work of law enforcement. Mr. Trump's decision was well within his authority, but well beyond the norms of U.S. politics.

Now, the American system and Mr. Trump face a series of tests – and the result could determine whether U.S. democracy begins to slide in an illiberal direction.

Mr. Trump must nominate a replacement for Mr. Comey and he can select either a loyalist or someone with a reputation for independence. Members of Congress will decide whether to intensify the Russia investigation. And voters will determine how much any of this matters to their views of the President and his party.

Republicans will play a crucial role. Some lawmakers have voiced misgivings about Mr. Trump's decision to fire Mr. Comey, but very few have called for bolstering the Russia investigation.

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Mr. Trump's popularity among the party's core voters remains considerable, giving lawmakers little electoral incentive to distance themselves from the President. Behind the Republican united front, however, discomfort is growing.

In a single week, Mr. Trump fired Mr. Comey, linked the dismissal to the continuing Russia investigation, issued a threat to the former FBI director over leaks to the press and alluded to the possibility that the President's conversations were being recorded.

For some political scientists and historians, the situation represents not so much a crisis but an inflection point: a moment that will either be remembered for the way lawmakers and the public reasserted clear boundaries on presidential behaviour, or as the time when those boundaries began to erode in important ways.

Mr. Trump said the Russia investigation played a part in his decision to dismiss Mr. Comey. "When I decided to just do it, I said to myself, I said: 'You know, this Russia thing, with Trump and Russia is a made-up story,'" Mr. Trump told NBC on Thursday. That assertion contradicted the earlier rationale for the firing provided by the White House, which pointed to Justice Department criticisms of Mr. Comey's performance.

On Friday, Mr. Trump went further, possibly reacting to a report in The New York Times that he had invited Mr. Comey to dinner in January and asked the FBI director to pledge his loyalty. "James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!" Mr. Trump wrote on Twitter. Even for Mr. Trump, it was an astounding move, an apparent threat to the former FBI director.

Republicans, meanwhile, have seen their legislative agenda disrupted by a controversy of Mr. Trump's making. "You see a tremendous number of House representatives who are sort of in hiding right now," said Zach Wamp, a Republican who represented a district in Tennessee for eight terms.

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"They're hunkering down to see exactly how difficult the storm is."

Mr. Wamp said that Mr. Comey's firing reflects "at a minimum, poor timing," but by itself will not erode Mr. Trump's support in a Republican-controlled Congress. However, if further investigation shows "any attempt to cover things up, or to try to take actions to help the administration stay out of trouble," then Mr. Trump's backers in Congress would distance themselves from him quickly, Mr. Wamp said.

Mr. Trump's firing of Mr. Comey is unlikely to shake his base of support. The way Mr. Comey was fired was "done poorly," said Aaron Avery, a dental student in Kansas who voted for Mr. Trump. But "I generally like where we're going with him."

Randy Proctor, a bus driver from Pennsylvania and a Trump supporter, said the situation with Mr. Comey was "no different from a regular job." If Mr. Trump didn't have "faith and confidence" in the FBI director, Mr. Proctor said, "You can't keep him."

Mr. Trump's supporters are also influenced by the portrait of Mr. Comey's firing in conservative media outlets, where the move has met with approval.

"I would not expect anything Donald Trump does to turn off his base until you see the right-of-centre media turn against Donald Trump," said Chris Jackson, vice-president of Ipsos Public Affairs, a polling firm in Washington.

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However, the same is not true of independent voters, whose support for Mr. Trump has ebbed since he took office.

Voters will not have their say until the midterm elections in 2018. In the meantime, the fallout from the Comey firing rests with Congress. Republicans are "in a wait-and-see-mode," said Frances Lee, a political scientist at the University of Maryland who studies Congress.

If the President nominates a close ally to the post of FBI director – for example, New Jersey Governor Chris Christie – Prof. Lee believes that some Republicans would join Democrats in objecting.

Presidential historian Barbara Perry said Mr. Trump has shown himself to be a leader unprecedented in the history of his office. "We have never had a genuine demagogue in this authoritarian style."

Historically, presidents have turfed high-level officials over policy disagreements, or ethical lapses (William Sessions, the FBI director ousted by Bill Clinton for billing holidays and home renovations to taxpayers), or for embarrassing the government (Donald Rumsfeld, ejected as defence secretary after the war in Iraq went off the rails).

But the only parallel to Mr. Trump's firing of Mr. Comey was Richard Nixon's "Saturday Night Massacre," said Prof. Perry, director of presidential studies at the University of Virginia's Miller Center. Mr. Nixon pushed out Archibald Cox, the special prosecutor investigating the Watergate scandal. Both his attorney-general and deputy attorney-general resigned rather than carry out the firing of Mr. Cox.

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For experts who study how democracies can slowly become less democratic, Mr. Comey's firing is a signal moment. Mr. Trump has already demonstrated a lack of commitment to certain norms through his rhetoric – threatening to lock up his election opponent, casting aspersions on judges and attempting to intimidate the media. But until dismissing Mr. Comey, they say, he hadn't actually acted on such talk.

"Maybe [Mr. Comey's firing] turns out to be an aberration," said Yascha Mounk, a lecturer at Harvard University who researches how democracies can erode from within.

"Or maybe this is the beginning of an assault on any kind of mediating institution that is capable of checking executive power in this country. Let's not declare victory long before we know."