With the House impeachment inquiry into President Biden expected to resume shortly after the election of a new speaker, House Republicans are zeroing in on a newly uncovered mountain of documentary evidence they say proves that Vice President Biden cooperated closely with his troubled son’s firm and its successful efforts to profit off his father’s office. 

Republican lawmakers are pointing to this latest document dump as overwhelming proof that, despite the president’s repeated protestations to the contrary, Mr. Biden and his office were active partners in the first son’s business ventures. Not only was Hunter Biden’s firm in near-constant e-mail contact with the vice president’s office, the e-mails show, he also accompanied his father on foreign trips to countries where he would do business deals, appearing by his father’s side at official ceremonies.

A former aide to President Trump, Stephen Miller, won access to the e-mail records when his organization, America First Legal, filed a Freedom of Information Act request. His request unearthed a trove of nearly 20,000 emails that were exchanged between the then-vice president’s office and the younger Mr. Biden, his business partners, and the Lion Hall Group, the firm set up by the president’s brother, James, who also did consulting work trading heavily on the Biden name. 

In March, America First Legal also released emails showing that President Obama’s National Security Council had serious concerns about the younger Mr. Biden joining the board of a Ukrainian energy company, Burisma, for which the first son was paid more than $80,000 a month until his father left the vice presidency, at which time his salary decreased dramatically. 

This comes just weeks after the federal government acknowledged that the president had used multiple private email addresses with fake names to communicate with his son and other family members while he was vice president. 

In August, the National Archives confirmed the existence of about 5,400 emails in which Mr. Biden used the pseudonyms Robin Ware, Robert Peters, and JRB Ware to send and receive information, including about government business. The emails are being sought by the Oversight Committee, which is investigating the younger Mr. Biden’s tangled business affairs, as well as by another conservative legal group, the Southeastern Legal Foundation, which on August 28 filed a lawsuit seeking the emails’ release. 

The president’s claims that he “knew nothing” about his son’s foreign business dealings were also contradicted by a former associate and friend of the first son, Devon Archer. 

Archer told the House Oversight Committee in a closed-door July meeting that the elder Mr. Biden called into the first son’s business meetings “at least” 20 times, though Archer confirmed that he personally never heard the two Bidens discuss business matters. 

The impeachment inquiry into the president — which has been delayed by the absence of a House speaker — has quickly focused on the first son, his foreign business dealings, and his father’s alleged involvement. 

At the first public hearing of the impeachment inquiry, the chief investigator of the Biden family, the House Oversight Committee chairman, Congressman James Comer, highlighted records his panel received showing that members of the Biden family had received tens of millions of dollars originating overseas during Mr. Biden’s time as vice president. 

“Since assuming our Republican majority in January, the House Oversight and Accountability Committee has uncovered a mountain of evidence revealing how Joe Biden abused his public office for his family’s financial gain,” Mr. Comer said in his opening statement.

“For years, President Biden has lied to the American people about his knowledge of and participation in his family’s corrupt business schemes,” Mr. Comer added. “At least 10 times, Joe Biden lied to the American people that he never spoke to his family about their business dealings.”

This also comes as the younger Mr. Biden is facing the possibility of federal prison time. He was indicted by a federal grand jury in September on three felony counts related to his purchase of a handgun in 2018, when he was in the throes of a crack cocaine and alcohol addiction. He owned the gun for a few days before his girlfriend, Hallie Biden, the widow of his brother Beau, found the Colt .38 Special in his truck and threw it out in a garbage bin outside a grocery store at Wilmington, Delaware, leading to the weapon being discovered by law enforcement. 

The latest development will likely end any chance Mr. Biden has to end the trial before it begins, with a federal judge at Delaware dismissing a gun charge for which the first son was going to enter a probationary period before the same judge killed that plea deal. 

Judge Maryellen Noreika, who is set to preside over the coming criminal trial, dismissed the charge on Wednesday following a request by the prosecution. Special Counsel David Weiss made the motion to dismiss so the criminal trial could move forward. Mr. Biden’s attorney had previously argued that the criminal trial should not take place because the prosecution had signed the plea agreement in July.