When Gary Shapley, a longtime I.R.S. investigator, wanted to air his accusations that the federal government had mishandled the tax investigation into President Biden’s son Hunter, the information he had was so sensitive he couldn’t even provide it to his own lawyer without potentially committing a felony.

So through his attorney, he approached Empower Oversight, a small group mostly composed of Republican lawyers with deep experience in Capitol Hill investigations — including years spent as aides to Senator Charles E. Grassley, Republican of Iowa — who coached him on what to do.

Armed with intricate knowledge of Congress’s byzantine procedures and various whistle-blower statutes, some written by Mr. Grassley decades ago, the men developed a strategy for how to get the information to Congress lawfully.

Months later, Mr. Shapley and a fellow I.R.S. investigator, Joseph Ziegler, were testifying in open session before the House Oversight and Accountability Committee, detailing their claims that the president’s son had received preferential treatment, which many in the Republican Party eagerly publicized as proof of their frequent accusations that President Biden and the Justice Department are corrupt.

It was not the first time that Empower Oversight had played a key role in facilitating some of the many Republican investigations into Mr. Biden’s family, his administration, and federal law enforcement. As the G.O.P. presses forward with inquiries aimed at uncovering wrongdoing by the president, the group has become a critical part of the Republican investigative ecosystem, using its knowledge of Capitol Hill to shepherd through Congress witnesses who can put names, faces and crucial details to the allegations being made.

In addition to the I.R.S. whistle-blowers, Empower Oversight also represents two F.B.I. agents who have harshly criticized the agency in appearances before the Republican-led select House committee on the weaponization of government.

House Republicans have made no secret their goal is to use their investigative powers to scrutinize President Biden and defend former President Donald J. Trump from the myriad criminal cases he faces. They’ve had no shortage of help from outside groups, such as the Heritage Foundation and the Conservative Partnership Institute, which bring in tens of millions in contributions a year to fuel conservative causes. Empower Oversight operates on a much smaller budget and pays salaries to just two employees.

Despite the outside help, the efforts of House Republicans have rarely produced clean hits during this Congress, and they have struggled to surface proof of any misconduct by the president despite months of effort. Some of the whistle-blowers who have appeared on Capitol Hill have had their security clearances revoked, and have been criticized by Democrats as aggrieved former officials who are being backed financially by Mr. Trump’s allies.

Others have been more problematic. One highly touted potential Republican witness turned out to be a fugitive from justice who was indicted on charges of arms trafficking and working corruptly for foreign countries.

Getting Mr. Shapley and Mr. Ziegler before Congress was a tricky task because unauthorized disclosure of tax information is a felony punishable by up to 5 years in prison. Members of the Empower Oversight team asked lawmakers from both parties to deputize two of Mr. Shapley’s lawyers to act as agents of the legislative branch, allowing them to review Hunter Biden’s private tax information.

Cognizant of the legal boundaries, Mr. Shapley and his lawyers were careful never to say Hunter Biden’s name publicly during that time.

House Republicans then arranged for Mr. Shapley to testify behind closed doors before the Ways and Means committee, the only panel in the House to which he could legally report such information. Using the same process as Democrats did when they released former President Donald J. Trump’s tax returns, Representative Jason Smith, a Missouri Republican who won the chairmanship of the committee in part by pledging to investigate Hunter Biden’s taxes, then led a contentious vote to make the allegations public.

The result was perhaps the most impactful testimony from any of the witnesses the G.O.P. has produced this Congress. Mr. Shapley claimed that Justice Department officials had stymied and slow-walked the investigation, and produced text messages, obtained through a search warrant, in which the younger Mr. Biden appeared to have engaged in a shakedown of a Chinese business associate by claiming that his father was sitting next to him awaiting confirmation of a deal.

The testimony prompted hours of unflattering media coverage of Hunter Biden, including a grilling of the White House press secretary about President Biden’s ties to the situation.

It also drew fierce pushback from Democrats, Justice Department officials and Hunter Biden’s legal team, which accused the Republican-led Ways and Means committee of releasing materials that “violated the spirit, if not the letter, of the tax laws and federal rules governing investigations” and told the Justice Department that Mr. Shapley has broken federal laws.

“It is no secret these interviews were orchestrated recitations of mischaracterized and incomplete ‘facts’ by disgruntled agents who believed they knew better than the federal prosecutors who had all the evidence as they conducted their five-year investigation of Mr. Biden,” Mr. Biden’s lawyer, Abbe Lowell, wrote to the Ways and Means committee.

Representative Sheila Jackson Lee, Democrat of Texas, railed against Empower Oversight, arguing that its role suggested that the charges were part of a partisan operation to tear down the president.

“Here is what I think is the most interesting piece of this whole puzzle,” she said at a recent Judiciary Committee hearing, noting that Empower Oversight is run by former Republican staffers. “Does anyone need any further proof that these allegations are ginned up, corrupt political stunts advanced by those who don’t want to see us follow the law?”

Empower Oversight rejects the suggestion its work is partisan in nature. While its leaders are Republicans, they say their job is to offer legal advice and support to whistle-blowers who approach them.

Tristan Leavitt, the group’s president and a former aide to Mr. Grassley and other Hill Republicans, said his organization’s “nonpartisan work stands for itself.”

Although Mr. Leavitt worked on the House inquiry into Operation Fast and Furious, a disputed gun trafficking investigation during the Obama administration, he has worked under a Democratic staff member, given presentations to left-leaning groups, and earned praise from other whistle-blower rights advocates. Empower Oversight is also a member of the Make It Safe Coalition, a group of organizations representing whistle-blowers across the political spectrum.

One of the I.R.S. whistle-blowers the group represents said he is a Democrat.

“We are lawyers and subject matter experts offering help to people who need it. We don’t ask them their partisan affiliation — it’s irrelevant,” Mr. Leavitt said in a post on Twitter. “The merits of the case are what’s relevant.”

Empower Oversight was started in 2021 by Jason Foster, a former aide to Mr. Grassley known for his aggressive tactics investigating the Justice Department. Mr. Foster was the subject of a ProPublica article that detailed posts he made in the mid-2000s on an anonymous blog under the name “Extremist.” ProPublica reported that they included posts “expressing worry about a Muslim takeover and whether Joe McCarthy got a bum rap.”

The blog has since been taken down, and Mr. Foster apologized, saying his “pen name” had been satirical and that his writings had been “stupid and wrong.”

In an interview, Mr. Foster said he started Empower Oversight because wanted to “help people make protected disclosures in the right way.”

“People can make all kinds of mistakes and it can cost them tremendously, even when they have the right motivations,” Mr. Foster said. “Exactly when, where and how you disclose something is often not well understood. We want to help them do it the right way and follow the rules.”

Much of the group’s appeal to whistle-blowers stems from the men’s experience working for Mr. Grassley, who during his 42 years in the Senate, became a magnet for them. While Mr. Grassley and his staff were known to investigate both Republican and Democratic administrations — he targeted the Reagan defense budget in the 1980s and helped expose the F.B.I.’s crime lab scandal in the 1990s — in recent years Mr. Grassley has become intensely focused on Hunter Biden’s international business dealings.

On Thursday, he released a document obtained from a whistleblower containing unverified allegations that both Hunter and President Biden had accepted bribes. The F.B.I. accused Mr. Grassley of risking “the safety of a confidential source” by releasing the document, and Lev Parnas, an associate of Mr. Trump’s personal lawyer Rudolph W. Giuliani, who was tasked with digging up dirt on the Bidens in Ukraine, said Mr. Grassley was spreading “conspiracy theories.”

A former top investigator for Mr. Grassley recently urged him in an op-ed in The Hill to abandon his current “political battles” and return to his roots.

Empower Oversight is not the only group on the right that has jumped in to assist Republican-led investigations on Capitol Hill. Kash Patel, who served as an aide to Mr. Trump, has provided money or covered legal fees for some witnesses who have come forward to testify to the weaponization panel about alleged abuses inside the Federal Bureau of Investigation.

And a former F.B.I. agent turned critic of the department, Kyle Seraphin, has cut checks for more than $255,000 to two of the witnesses with the subject line “Hold the Line.”

One of Empower Oversight’s clients, Marcus Allen, has been offered money by Mr. Seraphin but has yet to accept it, because he is awaiting legal guidance as to whether he can do so while remaining employed by the F.B.I., his lawyers said.

Representative Jim Jordan, Republican of Ohio and the chairman of the Judiciary Committee who is leading the weaponization inquiry, has said the payments were needed so the men could feed their families after being disciplined by the bureau.

The Heritage Foundation, a conservative think tank, also has launched a group dedicated to aiding the Republican investigations. Its latest actions were to file a motion attempting to force the Justice Department to turn over records about the Hunter Biden investigation in an attempt to derail his plea agreement.

The Conservative Partnership Institute, which counts Mark Meadows, the former Republican congressman who served as Mr. Trump’s chief of staff, among its leaders, held oversight trainings for Republican investigators. The Center for Renewing America, run by Russell T. Vought, Mr. Trump’s influential former budget director, employs one of the witnesses against the F.B.I. And Marco Polo, headed by Garrett Ziegler, a former aide in Mr. Trump’s White House, has been publishing material from Hunter Biden’s laptop and was poised to enter his child support case.

To be sure, there is a well-financed array of groups on the left that have sprung up to push back on the Republican-led investigations. Facts First USA, run by the Democratic activist David Brock, says it has a $5 million-a-year “SWAT team” focused on defending the Biden administration and Hunter Biden. The Congressional Integrity Project, which is associated with SKDK, a political consulting firm with close ties to the Biden White House, has a team of 15 people who have spent months countering the Republican investigations and attacking Mr. Trump.

“One of the biggest successes of this organization has been to unify the Democratic message against the failed investigations,” said Kyle Herrig, the director.

Christopher J. Armstrong, a former chief oversight counsel for Republicans on the Senate Finance Committee, noted that some of the Republican investigations have been harmed by overstatements, hyperbole and wild accusations.

But he said the I.R.S. whistle-blowers have generally presented themselves as sober and serious, with firsthand knowledge of the events they’re describing.

“What’s harmed oversight of the Biden administration is the vast amount of hucksters that are out there,” Mr. Armstrong said. “At the same time, I think there are real professionals who are doing amazing work.”