It's been a week of political turmoil in Washington, as US President Donald Trump faces a backlash for sharing sensitive intelligence with Russian officials.

Democratic lawmakers are calling for Mr Trump to be impeached — but is impeachment even possible, let alone likely?

The ABC asked a professor of law at George Washington University, Jonathan Turley, this very question.

Professor Turley testified at Bill Clinton's impeachment hearings 20 years ago and was the lead defence attorney during the most recent impeachment trial of a federal judge.

What exactly is impeachment?

Impeachment is the process by which Congress can bring and prosecute criminal charges against a sitting president.

It requires first a majority vote in the House of Representatives and if successful can lead to the president's removal.

Richard Nixon faced impeachment in the wake of the Watergate scandal of the 1970s. ( Supplied: Richard Nixon Presidential Library and Archives )

Two presidents have been impeached in US history, Andrew Johnson and Bill Clinton, both of whom were subsequently acquitted.

Richard Nixon famously faced impeachment proceedings related the Watergate scandal of the 1970s, but resigned before they could begin.

Nevertheless, the US Constitution was designed to make it very difficult to remove a president from office, Professor Turley says.

"[The system] guarantees that presidents can take unpopular decisions — if Lincoln was subject to a parliamentary system, he probably wouldn't have made it through the Civil War," he said.

"Impeachment is a standard that demands a level of great caution and circumspection, but history has shown that both houses have reserved that process for the most extreme circumstances."

Does Trump qualify for impeachment?

In short, not yet.

Mr Trump can attract as much controversy as he likes for his decisions while in office, but only some sort of serious illegality will lead to potential impeachment.

"The standard for impeachment of high crimes and misdemeanours requires substantial improper conduct," Professor Turley says.

After all, Mr Trump has previously complied with court rulings against his travel ban as well as his confrontation with so-called 'sanctuary cities'.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 10 minutes 6 seconds 10 m Listen to the full interview with Jonathan Turley on The World Today Download 18.5 MB

"When Trump fired Comey, he had authority to do that. None of this makes his actions correct, but they do make his actions legal," Professor Turley says.

But if the President defied the Supreme Court, Professor Turley says, that would cause a constitutional crisis to create grounds for impeachment.

So why are so many calling for it?

Calls from Democratic lawmakers for Mr Trump's impeachment have amplified since Monday, as well as from others like Bob Baer, a former CIA case officer for the Middle East.

"He's not fit in so many ways and clearly he doesn't understand the Department of Justice is supposed to be independent, and the FBI as well," Mr Baer told The World Today.

"What other choice do you have but impeachment?"

But according to Professor Turley, the calls for impeachment are misguided.

"That's part of the problem, that many are viewing impeachment as a way to end what they view as disastrous presidency — that's not what it's designed for, and it's also not what it can be used for," he says.

"There's a tendency in Washington today to have what we call in Virginia 'buck fever'.

"It's when hunters go out and just shoot at anything that moves, and you sort of see that in some of these calls — we're just not there yet, as hopeful as people may be."

Nevertheless, Mr Trump has opened himself up to criticism over a perceived hypocrisy in his comments about Hillary Clinton's email controversy before the election.

"Hillary Clinton was reckless in the use of classified information, but certainly this is an instance where Trump appears to be equally reckless, if not more so," Professor Turley says.