As if the impeachment trial taking place south of the border weren't alienating enough, with its arcane protocols, dense arguments and predictable partisanship, there is now a legal battle raging outside the Senate chamber that has the country's leading constitutional experts taking to Twitter and the op-ed pages.

High-profile defence lawyer Alan Dershowitz, known for his celebrity clients, such as O.J. Simpson and Mike Tyson, lobbed a verbal grenade into the Senate trial of U.S. President Donald Trump this week whose blast radius could reach far beyond the impeachment proceedings.

When asked whether it mattered if there was a quid pro quo in Trump's withholding of aid to Ukraine last August at the same time as he was asking the country to investigate his political rival, Dershowitz told senators that if a president believes his re-election is in the national interest, it's not wrong for him to do something to benefit his own electoral interest.

"If a president does something which he believes will help him get elected, in the public interest, that cannot be the kind of quid pro quo that results in impeachment," he told senators Wednesday in the question and answer portion of the trial.

Those words stunned at least the Democratic side of the Senate chamber and a good portion of the legal scholars watching and resonated well into Thursday evening.

The president as king

Dershowitz's claim was "a descent into constitutional madness," the Democrats' lead prosecutor, Adam Schiff, told senators.

Cable news pundits talked of "a march to monarchy."

Dershowitz, already at odds with legal consensus over his view that abuse of power does not fall under the high crimes the U.S. Constitution considers impeachable, earned scorn from both sides of the political spectrum.

Adam Schiff, who is the lead Democrat prosecuting the impeachment case in the Senate, said the idea that a president can do what he wants as long as he believes it's in the national interest is 'constitutional madness.' (U.S. Senate TV/Reuters)

"Where his definition of impeachable offences would proscribe too little, this argument would protect too much in presidential misconduct," said Jonathan Turley, a legal scholar at George Washington University and the sole expert witness the Republicans called in the House impeachment hearings who has criticized the impeachment as "rushed and slipshod."

"The argument becomes circular: presidents cannot be impeached for politics, but he argues all is politics for presidents," he wrote in an opinion piece for USA Today.

George Washington University Law School professor Jonathan Turley doesn't think Trump should be impeached but doesn't agree with Alan Dershowitz's interpretation of what a president can do to further his own electoral prospects. (Jacquelyn Martin/The Associated Press)

Even some of those who think Trump may have had a legitimate reason to hold up military aid to Ukraine and solicit co-operation with investigations into his political opponent fear Dershowitz's defence of the president could pave the way for an expansion of executive power that could allow Trump and future presidents to pursue political opponents with impunity.

"That is an end-will-always-justify the-means argument that I find troubling," said John Malcolm, vice-president of the Institute for Constitutional Government at the Heritage Foundation in Washington D.C.

Dershowitz responds in tweetstorm

Dershowitz tried to address the firestorm in an epic 20-tweet-long thread in which he attempted to clarify his point.

"Presidents cannot do anything illegal in order to get re-elected. But nor can their lawful behavior be turned into a criminal or impeachable offense just because it was motivated in part by electoral considerations," he wrote.

I did not say or imply that a candidate could do anything to reassure his reelection, only that seeking help in an election is not necessarily corrupt, citing the Lincoln and Obama examples. Critics have an obligation to respond to what I said, not to create straw men to attack. —@AlanDersh

His critics were not appeased, saying such a defence will embolden a president who already feels the U.S. Constitution grants him absolute immunity through his position as president and has used that justification to defy congressional investigation into his Ukraine dealings.

"The two big checks on the president are congressional oversight and impeachment, and they would have been destroyed," said Allan Lichtman, a presidential historian who argued in favour of Trump's impeachment in his 2017 book The Case for Impeachment.

"Dershowitz has given him cover to do that. … This could not be more significant."

The Nixon parallel

Comparisons with Richard Nixon's abuse of presidential power abounded in media coverage of the controversy. The 37th U.S. president who, facing impeachment, resigned in 1974 over his role in the Watergate political scandal, told a journalist who asked him why he sanctioned an intelligence operation that required burglary, "When the president does it, that means it is not illegal."

U.S. President Richard Nixon resigned before he could be impeached in 1974. (The Associated Press)

It's an imperfect comparison, says Corey Brettschneider, author of The Oath and the Office: A Guide To the Constitution For Future Presidents and a professor at Brown University.

"He, of course, did say that famous line … but he didn't create a whole philosophy around it or defend himself after Watergate in that way," he said.

"I don't think Trump has an understanding ... of the law as a constraint on his behaviour in the way that Nixon did."

Trump exploded realm of executive authority

Executive power has expanded under both Republican and Democratic presidents in recent decades, said Jessica Levinson, a professor at Loyola Law School in Los Angeles, but not to the extent that it has under Trump.

"We've never seen anything like this where you try and ... basically explode the realm of executive authority and use it for personal gain."

She says that while Dershowitz may have been roundly criticized by his colleagues for his interpretation of the constitution, that doesn't mean his argument won't have traction, especially if it's part of a winning legal defence that results in acquittal, as it's expected it will.

U.S. President Donald Trump, shown at a rally in Des Moines, Iowa, Thursday, has repeatedly said he thinks Article II of the U.S. Constitution allows him to do whatever he wants as president. (Jonathan Ernst/Reuters)

"You do get some sort of direct or indirect stamp of approval when your argument is the winning one," Levinson said.

But Malcolm cautions that the public interest argument is not the be-all and end-all of Trump's defence.

"People are going to vote to convict or acquit the president based on the whole panoply of arguments that had been made by both sides," he said.

Still, if lawyers rely on the "whatever is good for me is good for the country" defence in future cases to defend the president, the line between political and policy decisions could slowly be erased.

"I don't think that happens in one or two administrations, but I think it is possible that if this line of thinking just becomes accepted, then it really does dissolve," said Jessica Levinson.

Trial could end in acquittal Friday

Against the sound and fury of the Dershowitz controversy, the trial continued to barrel along Thursday and looked increasingly likely to end Friday with a vote to acquit the president without hearing from additional witnesses after Sen. Lamar Alexander, one of the Republicans thought to be considering voting with fellow moderates Lisa Murkowski, Susan Collins and Mitt Romney, confirmed he would oppose the calling of additional witnesses.

Closing arguments were set to begin Friday at 1 p.m. ET, followed by a vote on witnesses and possibly a vote on removal from office.