Donald Trump was acquitted of high crimes in his impeachment trial Wednesday - but could the unpredictable president now decide he's above the law?

The question hangs over a nervous, divided Washington.

History books will record that the Republicans' Senate majority delivered not guilty verdicts on the two charges.

For Trump, those votes are vindication of his insistence that he did nothing wrong in trying to get Ukraine to dig up dirt on Democratic election rival Joe Biden.

But even several Republicans concede that Trump's behavior was wrong, if not impeachable. One of them, Mitt Romney, broke ranks to convict.

And with the folding up of the impeachment court, Trump's critics say the unchastened president is now likely to abuse his office without fear.

"He will not change," said Adam Schiff, the Democratic congressman who led the impeachment case. "A man without character or ethical compass will never find his way."

Rick Wilson, a Republican strategist opposing Trump, wrote a Daily Beast column Wednesday in which he imagined an "unhinged and unbound" Trump dreaming that "vengeance is coming."

Ever since his shock rise to power in 2016, Trump has relished breaking norms - or, critics say, laws.

That goes all the way from swearing in public to pushing the byzantine scheme in Ukraine against Biden that ultimately led to his impeachment.

Trump comes from a business background where the CEO is used to getting what he wants. As a reality TV show personality, he played the character of an omnipotent boss firing employees with a mere point of his finger.

With no political experience, he clearly chafed at the traditions, endless rules and multiple layers of security surrounding the presidency.

In the early days, reported journalist Bob Woodward, frantic aides would literally swipe damaging documents from Trump's desk before he could sign them, like a letter abruptly ending a trade agreement with South Korea.

One by one, though, those pushing back from inside the White House disappeared. If they didn't resign, like defense secretary James Mattis, they were fired, like national security advisor John Bolton.

Outside the White House, Trump likewise imposed his will.

A huge investigation into his commercial and political relationship with Russians - and allegations that his election campaign colluded with Kremlin operatives - ended in a whimper.

His impeachment, launched by Democrats after a whistleblower revealed the secretive machinations in Ukraine, also petered out.

Trump once again had proved himself stronger.

Republican Senator Susan Collins was one of several from her party who criticized Trump's actions with Ukraine. Her hope, though, is that the experience will temper his future conduct.

"I believe that the president has learned from this case," she said. "The president has been impeached. That's a pretty big lesson."

Another Republican senator, Joni Ernst, says Trump will likely "go through the proper channels" from now on when he makes foreign contacts, unlike during his Ukraine caper.

Schiff and many others on his side have no such confidence. "He has done it before and he will do it again," the Democrat said.

Maureen Dowd, a veteran New York Times columnist and often scathing Trump critic, labelled Trump a "Godzilla" ready to run wild.

"The Republican Party has now lost whatever control they could exert over this president, any oversight they could have. It's gone."

But John Mueller, a political science professor at Ohio State University, told AFP that Washington's political class should take a deep breath.

"I doubt that Trump will be more unbound," Mueller said.

"He's already run all over immigration and tariff rules, etcetera," he said. "He may have learned his lesson."