Trump’s presidency is under siege from all directions — and it can only end in impeachment: legal expert

President Donald Trump's presidency is in grave danger, according to one legal expert, but he said it would not likely topple from a single blow.

Benjamin Wittes, editor in chief of Lawfare and a senior fellow at the Brookings Institution, said the succession of damaging blows from special counsel Robert Mueller and the coming Democratic majority could fatally weaken Trump's presidency, but only if Congress acted to remove him.

"The underlying metaphors are wrong," Wittes wrote for The Atlantic. "There is no sudden bend in the path of the investigation. There is no house of cards. The dominoes will not fall if gently tipped. The administration is not going to come crashing down in response to any single day’s events. The architecture of Trump’s power is more robust than that."

"We need to stop thinking of it as a fragile structure waiting for the right poke to fall in on itself," he added. "Think instead of the myriad investigations and legal proceedings surrounding the president as a multi-front siege on a walled city that is, in fact, relatively well fortified."

He compared the investigations arrayed against the president to siege warfare, which aims to weaken the target over time until it could no longer withstand the threat from outside.

"Siege warfare is not a matter of striking precisely the correct blow at the correct moment at a particular stone in the wall," Wittes said. "It is a campaign of degradation over a substantial period of time ... Active conflict is an episodic, not a constant, feature of siege warfare; the enemy army can encamp outside the walled city and blockade it without firing a shot. Over time, the walls and defending forces become degraded to such a degree that the invaders are able to scale the walls and sack the city."

"No, Mueller and his forces are not a Mongol horde," he added, "but the Trump White House is very much under siege."

Wittes examines each front of that siege -- from Mueller investigates alleged Trump campaign collusion with Russia and perhaps efforts to obstruct justice in that case, to his attorney Michael Cohen's cooperation with the U.S. attorney’s office in the Southern District of New York, to various lawsuits filed against Trump and his administration, to perhaps the greatest threat of all.

"There’s [a] big new army marching on the Trump fortress, with an expected arrival of January 3, 2019," Wittes said. "The leadership of the new Congress has already promised to intensify oversight of this administration. Representative Adam Schiff, incoming head of the House Intelligence Committee, has hinted at a restart of that committee’s moribund investigation into Russian interference in the 2016 election. Other committees will be aggressive as well."

The castle's defenders are slipping away in the night, Wittes wrote, listing the attorneys who have left Trump's defense to an overworked and understaffed White House counsel's office -- but he said the presidency remained too well fortified to fall to a Mueller bombshell.

"When the walls are finally breached, how will we know that it really is the beginning of the end?" he said. "Here’s a hint: The big one will not be a legal development, an indictment, or a plea. It will be a political development — that moment when the American political system decides not to tolerate the facts available to it any longer."

"What does that look like?" Wittes added. "It looks like impeachment."