Don McGahn. Drew Angerer/Getty Images

Michael Flynn's resignation on Monday from his post as national security adviser to President Donald Trump, which comes after about three weeks on the job, has placed the spotlight on another White House official: Trump's top lawyer.

Multiple outlets reported Monday that former acting Attorney General Sally Yates and a senior career national security official told Donald McGahn, White House counsel, in late January about strong concerns related to Flynn's communication with a Russian ambassador.

Yates said she believed Flynn misled senior administration officials about his December phone call with Russian Ambassador Sergey Kislyak, during which Flynn reportedly urged Kislyak to not overreact to the latest round of sanctions imposed by President Barack Obama, who levied them in response to Russia's interference in the 2016 US election.

The acting attorney general warned that Flynn was perhaps vulnerable to Russian blackmail, sources told The Washington Post.

Flynn had told Vice President Mike Pence that he did not discuss the Obama administration's sanctions on Russia with Kislyak. Pence later repeated Flynn's claim in a CBS News interview.

But The Post reported that it was unclear what McGahn, who had served as Trump's top campaign lawyer, did with the information Yates and another official provided him.

"It's unimaginable that the White House general counsel would sit on it [and] not tell anybody else in the White House," David Gergen, who worked in the Nixon, Ford, Reagan, and Clinton administrations, said during a Monday interview on CNN.

"In every White House I've ever been in, this would go to the president like that," he added, snapping his fingers.

Michael Flynn. Drew Angerer/Getty Images

Jack Goldsmith wrote in Lawfare on Tuesday that the crucial question moving forward will be "what McGahn did with this information, and when, and why."

As Goldsmith says in his post, this is only the latest White House firestorm involving McGahn.

Others include the several ethical controversies that have surrounded the administration since Trump won the November election, and the botched rollout of Trump's executive order that banned travel to the US from seven majority-Muslim nations.

"There were early indications that McGahn ignored the usual protocols for ethics vetting of Cabinet officials," Goldsmith wrote. "Since then the problem has only grown worse and has drawn bipartisan ire. One wants to know what McGahn's role has been in ensuring (or not ensuring) compliance with relevant ethics rules, and (as several Democrat senators asked last week) what 'clear and specific steps the White House is taking to prevent further violations of government ethics laws by members of the White House?'"

On the travel ban, Goldsmith wrote that one of McGahn's responsibilities was to coordinate with multiple agencies on legal policies and that he should have been able to anticipate and correct any problems with the executive order before it was issued.

"And he should have advised the president after his first anti-Robart tweet, and after the other more aggressive ones, that the tweets were hurting the president's legal cause," he wrote, referring to US District Judge James Robart. "If McGahn did not do these things, he is incompetent, and perhaps we can attribute impulsive incompetence to the president. But if McGahn did do these things — if he tried to put the brakes on the EO, and if he warned his client about the adverse impact of his tweets — then he has shockingly little influence with the president and within the White House."

Goldsmith wrote that if the New York Times report about Trump being angry he "was not fully briefed" on the details of an executive order about the organization of the National Security Council and its principals committee is accurate, that too falls on McGahn.

Former US Ambassador Norman Eisen, who served as Obama's top ethics lawyer during his first term, summed up the problems facing McGahn.

"[Two] biggest Trump debacles so far were both in WH Counsel McGahn's wheelhouse: Muslim ban EO & Yates telling him about Flynn convos," he tweeted.

And MSNBC's "Morning Joe" host Joe Scarborough — whose words carry weight with Trump, as he is a frequent viewer of the program — tweeted that McGahn is as good as gone after the Flynn debacle.

"All the knives are out for him," Scarborough wrote. "I wouldn't be surprised if he's gone in a week."