Republicans, who have repeatedly tried to link the president to his son’s legal troubles but have yet to produce proof to back up their allegations of corruption, have seized on the recent testimony of two I.R.S. whistle-blowers as their strongest evidence to date.

Gary Shapley, the lead I.R.S. agent who oversaw the investigation into Hunter Biden, and an unidentified agent who worked with Mr. Shapley told the Ways and Means panel that the Justice Department refused to pursue any evidence that could have led to Mr. Biden being implicated in the tax case against his son; tipped off Hunter Biden’s team before critical steps were taken in the investigation; and allowed the statute of limitations to expire on some prospective charges.

Mr. Shapley provided emails and messages to support crucial portions of his account, which was backed up by a second I.R.S. agent. The New York Times has also independently confirmed a key allegation made by Mr. Shapley: that prosecutors in California denied a request from Mr. Weiss to pursue charges against Hunter Biden in that state.

But Mr. Shapley’s account is at odds with statements made by both Attorney General Merrick B. Garland and Mr. Weiss, who in a letter to Congress said he had ultimate authority over the case.

The interviews demanded by House Republicans could shine a light on whether Mr. Weiss did, in fact, have the ultimate charging authority or whether his powers were constrained, as the whistle-blowers say. And they could reveal whether someone is lying.

The Shapley testimony — which came with newly released summaries of WhatsApp messages that purportedly show Hunter Biden seeking millions of dollars from a Chinese associate while invoking his father — was given after House Republicans had investigated the president and his family for months.

The inquiry into Hunter Biden’s overseas business dealings has produced some of Republicans’ most inflammatory claims, including a years-old, uncorroborated accusation that the elder Mr. Biden, while vice president, accepted a $5 million bribe from the Ukrainian company Burisma that was later covered up by the Justice Department under President Donald J. Trump.

Democrats on the House Oversight Committee tried to debunk this claim on Thursday by releasing a transcript of testimony to Congress obtained several years ago. In an interview with an associate of Mr. Trump’s personal lawyer Rudolph W. Giuliani, Mykola Zlochevsky, the owner of Burisma, denied that he had “any contacts” with the former vice president while Hunter Biden served on his company’s board. He also denied that the elder Mr. Biden or his staff “in any way” assisted Mr. Zlochevsky.

Republicans produced no proof that the elder Mr. Biden accepted payments from foreign businesses or that he took any action as a result of Hunter Biden’s overseas work.

Republicans have for months been building toward impeaching multiple Biden administration officials.

Speaker Kevin McCarthy has pledged to add Mr. Garland to the list if Republicans determine that he lied about Mr. Weiss’s authority in the Hunter Biden investigation.

“Someone is lying,” Mr. McCarthy posted on Twitter on Wednesday. “If we find it is Biden’s Attorney General, Merrick Garland, we will start an impeachment inquiry.”

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