On the eve of Hunter Biden’s court appearance to enter into a plea deal for misdemeanor tax crimes that would allow him to avoid prison time, House Republicans and conservative groups sought to intervene in the case, urging a judge to throw out the agreement he reached with prosecutors.
The highly unusual legal maneuvering — which experts said was unlikely to succeed — illustrated the lengths that House Republicans and their allied groups have been willing to go to as they have tried to use Mr. Biden’s legal and personal troubles to inflict political damage on his father, President Biden.
Representative Jason Smith of Missouri, the chairman of the House Ways and Means Committee, filed a brief in Federal District Court in Wilmington, Del., where Hunter Biden’s plea deal is to be considered by a judge on Wednesday.
The committee has heard testimony from two Internal Revenue Service investigators who claim to be whistle-blowers and have told the panel that the younger Mr. Biden received preferential treatment from the Justice Department. Mr. Smith’s brief asked the judge to consider the testimony in deciding whether to approve the agreement.
Another brief was filed by the Heritage Foundation, the conservative research group, which has started an operation dedicated to aiding the Republican investigations into President Biden.
Hunter Biden’s lawyers tried to stop Mr. Smith from filing his brief, saying that he had no standing and that the materials included in the filing should be filed under seal because they contain confidential taxpayer information.
The judge overseeing the case, Maryellen Noreika, agreed to seal the filing, but not before The New York Times was able to obtain a copy. The brief argued that the plea deal was “tainted,” citing the testimony of the two I.R.S. officials.
“The situation here is not that the Justice Department exercised charging or plea negotiation discretion, but the presence of credible allegations that the investigation, charging decisions and plea negotiations were tainted by improper conduct at various levels of the government,” wrote Theodore A. Kittila, a lawyer who filed the brief on behalf of Mr. Smith.
Hunter Biden struck a deal with the Justice Department last month to plead guilty to two misdemeanor charges of failing to file his taxes on time in 2017 and 2018, and to accept terms that would allow him to avoid prosecution on a separate gun charge.
Republicans have been trying to link Hunter Biden’s international business dealings in Ukraine and China to his father, suggesting that as vice president the elder Mr. Biden used his office to help his son and his son’s business partners. But no evidence has surfaced implicating President Biden, who has always maintained that he kept his distance from his son’s business.
Republicans have more recently tried to make a case that Hunter Biden’s plea deal was marked by favorable treatment from the Justice Department in his father’s administration. That assertion has been rejected by Attorney General Merrick B. Garland and by the prosecutor who has overseen the case, David C. Weiss, the U.S. attorney in Delaware, a Trump appointee.
Christopher J. Clark, a lawyer for Hunter Biden, chastised the Republicans for trying to interfere in the court case.
“Most troubling is that you have sought to append to a filing on the public docket hundreds of pages of documents, many of which contain grand jury secret information and confidential taxpayer information,” he wrote to lawyers for the Republicans, calling their actions “baseless and abusive.”
Mr. Smith has previously urged Mr. Garland and Mr. Weiss to enter materials based on the testimony by the I.R.S. officials into the court file in an attempt to undermine the plea deal.
The Justice Department has not responded to the House Republicans’ demands to enter files into the court record, but said in a letter to Congress on Monday that it wanted to “protect law enforcement work from even the perception of political interference, including from Congress.”
Seamus Hughes contributed reporting.