Former Vice President Joe Biden criticized the White House Thursday for allowing a top aide to President Donald Trump to stay on despite apparent knowledge of his alleged history of domestic abuse.

That aide, White House staff secretary Rob Porter, resigned Wednesday after the allegations became public.

"I can’t explain it. It's long past due that he left," Biden told NBC News' Andrea Mitchell when asked about Porter's access to the president and to White House Chief of Staff John Kelly.

"I understand that he departed — if you just look at it from a perspective of one thing, the FBI didn't think he should get a security clearance," Biden said, commenting on reports that senior White House officials, including Kelly, were aware of the accusations against Porter, and that the FBI had not issued Porter a permanent top security clearance.

"I'm having enormous difficulty understanding how this White House functions," Biden added, calling the administration "controlled chaos."

Porter's two ex-wives, Jennifer Willoughby and Colbie Holderness, told DailyMail.com in a report published Tuesday that Porter was physically and verbally abusive during their marriages.

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Willoughby and Holderness each confirmed the accounts of their allegations to NBC News on Wednesday. In a statement announcing his departure, Porter denied the accusations, saying, "These outrageous allegations are simply false."

Porter could depart the White House as early as Thursday, a White House official told NBC News, despite Press Secretary Sarah Huckabee Sanders saying Wednesday that Porter would remain on to ensure a smooth transition.

Deputy White House Press Secretary Raj Shah called the allegations are "serious and disturbing" at a White House Press Briefing Thursday, but declined to confirm what details about the abuse allegations the administration had not been informed of during Porter's background check.

He also said that Kelly became "fully aware" of the allegations on Wednesday and was "shocked," though he did not specify what information beyond the images was new to the chief of staff. Shah said the president became aware of the allegations Tuesday night and did not know that Porter was using a temporary security clearance during his time in the administration.

“I think it's fair to say that we all could have done better over the last few days in dealing with this situation," he said.

Biden said he believed the White House must have known that the FBI had reportedly flagged Porter's security clearance because of the past spousal abuse allegations.

"Sure they knew ... look, the culture's changing, thank God, but not fast enough," Biden said, referring to the watershed #MeToo movement, which has seen women come forward to detail their experiences of sexual assault and harassment.

"The idea that this would happen in the Oval Office — now, I don't know — I was briefed on the way in that when the president found out about it he said 'go,' and if that’s the case, if the president never knew about it, then good for the president," Biden said.

Biden, speaking to NBC News at the Penn Biden Center for Diplomacy and Global Engagement, also weighed in on whether Trump should sit down with special counsel Robert Mueller, who is leading the government probe into alleged Russian meddling in the 2016 campaign, which includes examining any links between Trump's campaign and Moscow.

Trump has signaled that he is eager to sit down with Mueller under oath, although a later report indicated that his lawyers advised against an interview.

Biden said that, as a lawyer, he would not advise Trump to agree to speak with Mueller because "he does not tell the truth, he is not accurate in much of what he says." On the other hand, Biden said, the president has a responsibility to meet with Mueller.

"A president should be prepared to answer the questions about one of the most serious security breaches that's occurred in American history, Russia attempting to affect and impact on the American electoral process," Biden said.

And as for speculation that he might run for president himself in 2020, Biden said, "I have plenty of time to consider whether or not to run ... we have really qualified people out there."

"My calculation has never been 'Who's running, can I win, can I not win,' it's, 'Is this right for me to do?'" Biden added. "And the only thing that's right for me to do now is to try to stop this enormous erosion of the moral fabric that's at the hands of Donald Trump and the Republicans."