Former Trump adviser Roger Stone. (Photo: Diego M. Radzinschi/ALM)

Democrats vowing to investigate President Donald Trump's commutation of Roger Stone's prison sentence will face significant constitutional hurdles, especially after a Supreme Court ruling on the limits of congressional subpoenas, according to former House lawyers.

House Judiciary Committee Chair Jerry Nadler and House Oversight Committee Chair Carolyn Maloney said in a joint statement after Stone's commutation that they want White House counsel Pat Cipollone to brief them on the grant of clemency. Nadler added that his committee "will conduct an aggressive investigation into this brazen corruption."

Speaker Nancy Pelosi also called for legislation to prevent a president from issuing a grant of clemency to "an individual who is engaged in a cover-up campaign to shield that president from criminal prosecution."

On Friday, after this story was originally published, Nadler announced his committee would consider two pieces of legislation in response to Stone's commutation: One to require transparency on acts of clemency, and another that would, while the president is in office, stop the clock on the statute of limitations for crimes he may have committed.

"President Trump and his friend Roger Stone did what they said they would do. Stone misled federal investigators, intimidated a witness, and was convicted for obstruction of justice—but would not testify to the president's wrongdoing. In exchange, President Trump made sure that Stone will never spend a day in prison," Nadler said in a statement. "This quid pro quo is unacceptable. Congress must act."

But former congressional attorneys say, barring a possible constitutional amendment, Congress cannot place limits on pardons. That power is granted explicitly to the president under Article II. And without potential legislation behind an investigation, the House faces an uphill battle in trying to conduct the probe.

House Judiciary Committee Chairman Jerry Nadler.

As Chief Justice John Roberts wrote in the U.S. Court's recent decision on congressional subpoenas for Trump's tax records, Congress's investigatory "power is 'justified solely as an adjunct to the legislative process,'" and "is subject to several limitations."

"Most importantly, a congressional subpoena is valid only if it is 'related to, and in furtherance of, a legitimate task of the Congress.' The subpoena must serve a 'valid legislative purpose,' it must 'concern[] a subject on which legislation "could be had,"'" Roberts wrote.

Thomas Spulak, a former House general counsel and partner at King & Spalding, said in an email that Congress is "unable to place any limits on the president's authority to commute sentences and issue pardons."

Spulak also said he believes lawmakers wouldn't be able to glean more details on Trump's decision-making in handing down the commutation, including those he discussed it with, under executive privilege protections granted to the president's deliberative processes. "So I believe in this case, the president would have a valid privilege to use to refuse to cooperate, which would also extend to subordinates," he said.

That means the House will not be able to point to potential legislation reforming pardon powers if they choose to launch a formal investigation on the Stone commutation.

Stanley Brand, a former House general counsel, pointed to Congress's attempt to investigate pardons handed down by former President Bill Clinton, including ones issued on Clinton's final day in office to financier Marc Rich and his business partner Pincus Green, which were criticized by both Democrats and Republicans. Clinton issued dozens of other grants of clemency on the last day of his term, which he later defended in a New York Times opinion piece.

Committees in both the House and the Senate held hearings on the pardons, featuring testimony from the Justice Department's pardon attorney at the time, career official Roger Adams. The U.S. Attorney for the Southern District of New York, Mary Jo White, was also reported to launched a criminal investigation into the matter, but no indictments emerged.

Lawmakers at the time were warned by legal scholars against altering the pardon powers through a constitutional amendment. However, considering such a constitutional amendment would open the door for issuing such subpoenas.

Brand, who believes the hearings on the Clinton pardons were unconstitutional, said the committee could invoke its oversight of the Justice Department to examine the pardons from that angle, but questioned the effectiveness of the approach. He said House Democrats could also eye outside actors who may have been involved in the process, like figures who lobbied for Stone's clemency.

"I guess they could go there, but I don't think the claims would be stronger for those people," Brand said, noting that any issued subpoenas would still have to be based on relevant legislation.

Spulak also said lawmakers "could identify areas that on their face could appear to lie outside of the decision-making process to try to get at what went on."

"But in the end, in this case, the president might well have the better constitutional arguments.  But that doesn't mean they shouldn't try," he added.

Another pathway for the House is its appropriations power. Nadler has already introduced legislation to cut $50 million from the Justice Department's budget, in response to what he and other Democrats describe as increased politicization at the agency under Attorney General William Barr.

Gillian Metzger, a constitutional law scholar with Columbia Law School, pointed to the House's power of the purse as potentially playing a role in the response to the Stone commutation.

"Even accepting—it's debatable—that Congress cannot zero out funding for core presidential authorities like the pardon power, Congress still has substantial discretion over how much to fund and information about how the president is using the pardon power would seem relevant to appropriations legislation Congress might enact," Metzger said.

Barr is also set to appear before the House Judiciary Committee on July 28, where he is certain to face questions about the Stone commutation.