Momentum is building for a second special counsel to investigate the handling of the Trump-Russia investigation by senior officials at the Justice Department and the Federal Bureau of Investigation.

This week, two powerful House Republicans added their voices to a growing list of names: House Majority Leader Kevin McCarthy (R-CA) and Majority Whip Steve Scalise (R-LA).

The idea already had the support of more than a dozen lawmakers and four senators.

Last week, four senators — Senate Majority Whip John Cornyn (R-TX), Senate Judiciary Committee Chairman Chuck Grassley (R-IA), Senate Judiciary Crime and Terrorism Subcommittee Chairman Lindsey Graham (R-SC) and Sen. Thom Tillis (R-NC) — sent Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein a letter requesting a second special counsel.

The week before, House Judiciary Committee Chairman Bob Goodlatte (R-VA) and House Oversight and Government Reform Committee Chairman Trey Gowdy (R-SC) wrote Sessions and Rosenstein requesting one.

It followed a letter by 13 GOP lawmakers, including some who have been requesting one for months: Reps. Lee Zeldin (NY); Jim Jordan (OH); Claudia Tenney (NY); Francis Rooney (FL); Matt Gaetz (FL); Ted Budd (NC); Jody Hice (GA); Mark Meadows (NC); Scott Perry (VA); Paul Gosar (AZ); Andy Harris (MD); Louie Gohmert (TX); and Dave Brat (MI).

It appears Sessions is weighing the idea. Earlier this month, he said he would “consider seriously” a second special counsel, and revealed he has appointed a former official outside of Washington to look into lawmakers’ concerns.

DOJ Inspector General Michael Horowitz has been investigating the DOJ and FBI’s handling of the Clinton email and Trump-Russia investigations, but Republicans argue he is limited to interviews with personnel within the Justice Department.

A special counsel has grand jury subpoena authority, can obtain search warrants, and can prosecute crimes committed by persons inside or outside of the DOJ.

To appoint a special counsel, several conditions must be met: One, the attorney general or his deputy must conclude the criminal investigation of a person or matter is warranted. Two, there must be a conflict of interest for the Justice Department to conduct the investigation. Three, a special counsel must be in the public interest.

House Intelligence Committee Chairman Devin Nunes (R-CA) last month outlined in a letter to Sessions the crimes DOJ and FBI officials may have committed in using the infamous Trump dossier to obtain a surveillance warrant on former Trump campaign member Carter Page.

Last week, FBI Deputy Director Andrew McCabe was fired for talking to reporters about an ongoing criminal investigation and repeatedly lying to investigators about doing so, which may also be a crime.

Republicans also argue the Justice Department cannot credibly investigate itself on this matter. In one potential conflict of interest, Deputy Attorney General Rod Rosenstein signed off on at least one of four surveillance warrant applications on Page, as did FBI general counsel Dana Boente.

Margot Cleveland, lawyer and senior contributor to the Federalist, argues the DOJ inspector general’s office could also be subject to bias.

“The career employees in the inspector general’s office hold the same potential for political passions and prejudices that tainted the Justice Department and FBI’s Clinton and Russia probes, including the FISA warrant targeting Page,” she wrote recently.

It is not clear how Sessions would determine whether a special counsel is in the interest of the public, but based on Mueller’s appointment, it would depend on whether the public would have “full confidence” in the DOJ’s ability to investigate itself.

A Rasmussen poll in late January said a plurality of 49 percent of likely U.S. voters believe a second special counsel should be appointed to investigate the FBI, including 65 percent of Republicans polled. That was before the House Intelligence Committee released a memo outlining potential DOJ and FBI abuse of the Foreign Intelligence Surveillance Act.

Critics of a second special counsel argue it would interfere with the Mueller probe, and turn into an scenario of dueling probes.

National security lawyer Bradley Moss said that any second special counsel investigation “would almost certainly wind up overlapping with the efforts of the ‘first’ Special Counsel.”

“At least some of Mr. Mueller’s team (current or former) would be interviewed as part of the second inquiry, and that fact would require the diversion of Mr. Mueller’s resources to defend the integrity of his own investigation,” he said.

“Investigating the investigators before the original investigation is even complete is anathema to our system of oversight and democratic governance,” he added.

Peter Strzok and Lisa Page — two former members of Mueller’s team who played pivotal roles on the FBI’s Clinton email and Trump-Russia investigations, would almost certainly be interviewed by a second special counsel.

Current member and senior DOJ official Andrew Weissman, who praised then-Acting Attorney General Sally Yates for defying the president’s immigration order and attended Hillary Clinton’s election night party, could also be interviewed.

Some conservative legal experts oppose a second special counsel on principle, pointing to how out of control Mueller’s probe has become.

“The special counsel is a pernicious institution. We should never have them,” said former prosecutor Andrew McCarthy, senior fellow at the National Review Institute and contributing editor at National Review. He called Mueller’s probe “the most pernicious arrangement I’ve ever seen.”

“At least in all the other special prosecutor arrangements we’ve had, we knew what the crime was going in. Whereas here, Rosenstein did not cabin Mueller’s jurisdiction with a crime. He basically unleashed him to go find crime. So we don’t really have a control over what’s being investigated or know how wide a net Mueller is going to cast.”

“I know Rosenstein said he’s not an unguided missile, but he looks pretty unguided to me,” he added.

Some proponents argue a check on Mueller’s probe is a good thing.

There is already suspicion over how the Mueller probe began. Former FBI Director James Comey admitted during a hearing that he leaked to the media one of his memos, claiming the president had asked him to let Flynn go, in order to prompt a special counsel. The president has denied making such a request.

There are also suspicions of bias on Mueller’s team, which has a number of members who have donated to Democrats, including former President Obama and Clinton. One, Jeannie Rhee, has represented Clinton, the Clinton Foundation, and former Deputy National Security Director for Strategic Communications Ben Rhodes in the past.

“The credibility of the Mueller investigation will be in doubt unless we get to the bottom of the many serious questions regarding the FBI’s handling of their investigation of the Trump campaign, as well as their investigation into Hillary Clinton’s negligent transmission of classified material over her unauthorized email server,” said Scalise, according to the New York Times.  

The appointment of a second special counsel could also mollify the president, whose frustration over the Mueller probe is mounting. A recent report by Axios suggested Mueller is focusing on obstruction of justice, not collusion.

The president has not weighed in on whether a second special counsel should be appointed, but one of his lawyers has expressed support.

“I think it warrants a special counsel,” Jay Sekulow told FOX News. “I believe that’s what’s going to happen.”