Until this month, the impeachment drumbeat never sounded so loud. And the hits just kept coming.

Thump. 

A reported memo from former FBI director James Comey detailing how Donald Trump allegedly requested he end an investigation into his ousted national security adviser.

Thump.

The U.S. president's firing of Comey, who was overseeing another investigation of alleged Trump campaign ties with the Russians.

Thump. 

Trump's apparent confirmation of reports he shared classified intelligence about terrorism with Russian diplomats.

Thump, thump, thump.

Amid the battery of potentially damaging news stories, calls to bring impeachment proceedings against Trump are being amplified by demands that he should be impeached for the "high crime" of obstruction of justice.

The allegations concern Comey's memo about Trump pressuring him to end a probe into former national security adviser Michael Flynn, who resigned in February after it came to light that he lied to the vice-president about speaking with the Russian ambassador.

Republican Senator John McCain on Wednesday compared it to the 1970s scandal that ended the presidency of Richard Nixon, saying Trump's political wrongdoings were "reaching a point where it's of Watergate size and scale."

But does it mean Congress will move on a political remedy to remove Trump from office, deeming him no longer fit to carry out the duties of the presidency?

Constitutional and congressional scholars don't think so. Not as long as Trump's Republican base holds strong. 

Trump Coast Guard

During a commencement address at the U.S. Coast Guard Academy in New London, Conn., on Wednesday, Donald Trump said about himself: 'No politician in history, and I say this with great surety, has been treated worse or more unfairly.' (Susan Walsh/Associated Press)

While Trump has historically low polling numbers for a president (36 per cent, according to the latest Quinnipiac data), his support from his own party remains his salvation.

That Quinnipiac data gives him 82 per cent approval among Republicans.

Impeachment is not a criminal matter, but a constitutional device for the removal of a federal officer whom the political branches believe should no longer be in power. It is, by nature, fundamentally undemocratic.

"So the framers made it a very difficult undertaking. We would be undoing the result of a free and open election," says Steve Billet, with George Washington University.

"With impeachment, we'd be saying that maybe we didn't mean that [election outcome]. That's a serious undertaking when you basically overthrow your own government."

Impeachment crusaders alleging obstruction of justice point to Comey's Flynn memo as evidence which, if corroborated, would amount to a smoking gun. The Senate intelligence committee has requested the FBI turn over the documents.

No president has ever been impeached by his own party, notes Josh Huder, a senior fellow with the Government Affairs Institute at Georgetown University. "So this would be truly uncharted territory."

With Republicans controlling both chambers, Huder says Trump would have to lose major support from his base in order for congressional Republicans to start worrying about their own seats.

"If Republicans in Congress start to feel that [this] jeopardizes their re-election, or the GOP brand, or the majorities in the House and the Senate," says Huder, then Trump's support in the party would "evaporate," triggering "a more precipitous" dive toward impeachment.

Party support that dips into the 70 per cent range is "where landslide elections start to occur," Huder says. "If you're getting below 80 per cent of your own [party's] support, you're looking at a very bad election."

'When the people demand it'

Impeachment — which is like an indictment — would be the first step toward being booted from office. 

Initiating impeachment proceedings is reserved for the gravest abuses of power, including treason, bribery and the more open-to-interpretation offence of "high crimes and misdemeanours."

It was under the "high crimes" justification that Bill Clinton was impeached, on the grounds of obstruction of justice, in 1998 in connection with a consensual affair. 

Nixon also faced obstruction of justice charges, though he resigned before an impeachment vote. A president can be prosecuted in a criminal trial after leaving office, but likely not while sitting in office.

Impeachment is not synonymous with removal from office, but removal can't happen without impeachment.

(FILE) USA TRUMP LAVROV SECRETS

Trump speaks with Russia's Foreign Minister Sergei Lavrov, left, and Russian ambassador Sergei Kislyak, right, on May 10 — the day after James Comey was fired. (EPA)

"Impeachment will come when the people demand it. That's an essential part," says Allan Lichtman, the elections forecaster who predicts in his new book, The Case for Impeachment, that Trump will be impeached in his first term.

"You need to put all the evidence together; it all has to be put together in the context of everything else that's going on."

In Lichtman's view, that would include allegations of collusion with the Russians in trying to influence the 2016 presidential elections, the failure to fire Flynn immediately after the White House was warned he could be compromised by the Kremlin, and the timing of Comey's dismissal.

Trump admitted in an NBC News interview he terminated Comey while thinking of "this Russia thing."

Trial before the Senate

If the House votes in a majority for impeachment, the case would then move to the Senate, which requires a higher bar of two-thirds majority to trigger removal. The upper chamber then conducts a trial with the president, presided over by the chief justice of the Supreme Court, with House "impeachment managers" presenting their case.

Two presidents have been impeached after a majority vote in the House of Representatives: Bill Clinton in 1998 and Andrew Johnson in 1868. Both were acquitted in the final Senate vote.

It takes conviction on only one count, or "article of impeachment," to be ousted, says Susan Low Bloch, one of the 19 constitutional scholars who testified before House judiciary committee on Clinton's impeachment.

The Comey memo is "just one more piece of evidence" suggesting Trump "might have been trying to cover up" some transgression, Bloch says.

"What's needed now is some kind of independent investigation into whether he did obstruct justice."

And that came Wednesday evening, when the U.S. Justice Department appointed former FBI director Robert Mueller as a special counsel to oversee the probe into alleged ties between Trump's campaign and Russia.

As for Russian President Vladimir Putin's offer to release transcripts of Trump's Oval Office discussions with two Russian diplomats about possibly classified intelligence, such an unsolicited favour may not be so welcome.

"The more the Russians get involved in this, the worse it is," says Rajan Menon, an international relations expert with City College of New York. 

"I don't know if Putin's really trying to throw Trump a life-raft, but it's more like throwing him an anchor."