The man who could one day supervise special counsel Robert Mueller's Russia investigation received an ethics waiver from the Trump administration in April, a watchdog has found.

Noel Francisco, a longtime conservative lawyer who is the U.S. solicitor general, is next in line to oversee Mueller's team if Deputy Attorney Rod Rosenstein were to be fired or resign.

But the left-leaning Citizens for Responsibility and Ethics in Washington (CREW) announced on Friday it had found a previously undisclosed ethics waiver that could present obstacles blocking Francisco from taking over that responsibility.

CREW notes that Francisco's former law firm, Jones Day, represents President Trump's presidential campaign in the special counsel investigation, and that they still share financial ties, among other issues. The short April 24 waiver relieves Francisco from an ethics pledge he signed — per Trump's ethics executive order — that would preclude him from participating in any investigation in which Jones Day was involved.

CREW said it was "troubling" that this waiver is absent from the Office of Government Ethics's online list of all ethics pledge waivers for Trump political appointees serving outside the White House.

Furthermore, CREW said the signature on the waiver appears to be that of former White House counsel Don McGahn, who also had ties to Jones Day and the same obligation not to get involved in any investigation in which the law firm has a client.

"Notwithstanding these waivers, of course, Mr. Francisco should still recuse from the Special Counsel investigation if Mr. Rosenstein resigns or is fired," CREW wrote in an online post. "There is still the matter of the half a million dollars Jones Day owes him, and there’s his own involvement in the Trump campaign’s landing team to consider."

The White House did not immediately return a request for comment.

Francisco was confirmed in September 2017 as solicitor general, a role in which he conducts and supervises the government's litigation at the Supreme Court.

In September, his name came up amid speculation that Rosenstein might depart following a New York Times article that said Rosenstein talked about secretly recording Trump and invoking the 25th Amendment to the Constitution to oust the president after FBI Director James Comey was fired in May 2017. The deputy attorney general has denied about considering such measures, and subsequent reports say that he was joking or being sarcastic.