WASHINGTON - After vowing a smooth transition, President Barack Obama and Donald Trump are presiding over one of the most tumultuous transfers of power in US history.

Assassinations notwithstanding, American presidential transitions have generally been peaceful affairs. But that doesn't mean they have been smooth.

Andrew Johnson was effectively barred from attending Ulysses Grant's swearing in. Herbert Hoover and Franklin Roosevelt did not speak to each other on inauguration day.

Bill Clinton's staff removed the "W" keys from dozens of White House keyboards before George W. Bush moved in.

But few transitions have been as tempestuous as Obama's passing of the baton to Trump.

When the pair met in the Oval Office a few days after Trump's shock November victory, the tone was cordial enough.

Hailing an "excellent conversation," Obama said "it is important for all of us, regardless of party and regardless of political preferences, to now come together, work together, to deal with the many challenges that we face."

Quite a show of unity for two men who are as different as two politicians can be: Obama, a 55-year-old former law lecturer, is as systematic as Trump, a 70-year-old reality TV mogul, is impulsive.

But Obama's early strategy of flattering his way into Trump's good graces has melted away with each incendiary tweet.

In the span of a few weeks, the president-elect has picked fights with Mexico, China, Toyota, Lockheed Martin, the media, Arnold Schwarzenegger and the cast of "Hamilton."

He has also taken aim at Obama personally.

"Doing my best to disregard the many inflammatory President O statements and roadblocks.Thought it was going to be a smooth transition - NOT!" Trump tweeted in late December.

Trump infuriated the White House by offering a running commentary on Obama's final weeks, criticizing his decision not to veto a UN resolution on Israeli settlements and the transfer of prisoners out of Guantanamo Bay prison.

"Trump has incinerated the 'one president at a time' rule. His behavior during the transition has been just as erratic as we've come to expect on most matters" said Larry Sabato, head of the Center for Politics at the University of Virginia.

The president-elect "has been acting as though he was co-president-or maybe already president. It's a total break with tradition," said Sabato.

- Red tide -

But it is the scandal over Russian involvement in the election that has put Obama and Trump most sharply at odds.

The White House imposed sanctions on Moscow and released a steady drip of evidence that the Kremlin tried to put its hand on the electoral scale, culminating in an intelligence report that was shocking in its bluntness.

"We assess Russian President Vladimir Putin ordered an influence campaign in 2016 aimed at the US presidential election," the joint CIA, NSA and FBI report read.

"We also assess Putin and the Russian Government aspired to help President-elect Trump's election chances."

Fearing that assessment will forever put an asterisk by his historic victory, Trump has lashed out at US intelligence and appeared to sided with Russia in a way that is unthinkable for the White House.

Team Obama's comments about Trump have become increasingly pointed.

In a farewell address, Michelle Obama urged young Americans not to fear the future but fight for it.

"You cannot take your freedom for granted," she said. "You have to do your part to protect and preserve those freedoms."

"Don't be afraid. Be focused, be determined, be empowered," she said. "Lead by example, with hope, never fear."

In an interview with his former aide David Axelrod, President Obama himself went as far as suggesting that he could have beaten Trump in the election were he allowed to run for a third term - a remark the president knew would smart.

"The outgoing administration has made life more than usually difficult for the incoming administration," said David Clinton an expert in presidential transitions at Baylor University.

Obama may be right. His approval rating is around 55 percent according to Gallup, putting him in the league of Bill Clinton and Ronald Reagan as they departed.

Trump's approval rating is 43 percent, according to the Real Clear Politics average, notably low for someone who just won an election.

But that will be of little comfort to Obama with his signature policies -- from curbing emissions to the nuclear deal with Iran -- in such grave danger.

"Most of Obama's legacy is dead and gone, or will be in the coming weeks and years," said Sabato.

In a last-ditch bid, Obama has jumped back into the political fray with a series of interviews and speeches aimed at stopping Trump from rolling back landmark health care reforms.

On Tuesday in Chicago, he will deliver a farewell address that is set to focus as much on the future as on the last eight years.

The message for Trump is unlikely to be subtle.