The House select committee investigating the Capitol attack formally asked the Republican House minority leader, Kevin McCarthy, on Wednesday to cooperate with its inquiry into the January 6 insurrection, escalating the pressure on Donald Trump’s top allies in Congress.

The committee said in a letter to McCarthy that the panel is seeking details about his conversations with the Trump White House and the former president in the days leading up to and during the Capitol attack, as well as discussions in its aftermath.

“We also must learn about how the President’s plans for January 6th came together, and all the other ways he attempted to alter the results of the election,” Congressman Bennie Thompson, the chairman of the select committee, said in the letter.

Thompson said that McCarthy was of particular interest to investigators as he spoke to Trump directly as the former president’s supporters stormed the Capitol to stop the certification of Joe Biden’s election win.

The select committee’s request to McCarthy marks a significant political moment for the investigation and demonstrates their resolve to pursue testimony from the highest-ranking Republican in Congress as they examine potential criminal conduct by the former president.

It also set the stage for a bitter political showdown after McCarthy said late on Wednesday he would not cooperate with the investigation, accusing the panel of “an abuse of power” and of “not serving any legislative purpose” – an argument rejected by multiple federal courts.

Thompson said in the letter that the committee is, in the first instance, interested in McCarthy’s phone call to Trump on 6 January during which he unsuccessfully begged the former president to call off the pro-Trump mob as it stormed the Capitol in his name.

According to an account of that call presented at Trump’s second impeachment last year, the former president sided with the rioters and in refusing to take action, told McCarthy that they were evidently more upset about the election than the House Republican leader.

“You have acknowledged speaking directly with the former President while the violence was underway on January 6,” Thompson wrote. “This information bears directly on President Trump’s state of mind during the January 6 attack as the violence was underway.”

The chairman said that House investigators wanted to ask McCarthy about why he still objected to Biden’s election certification even after the Capitol attack took place, and even though he appeared to recognize that Trump was responsible for the insurrection.

“The select committee wishes to question you regarding communications you may have had with President Trump, President Trump’s legal team, Representative [Jim] Jordan, and others at the time on that topic,” Thompson wrote.

The select committee’s request to McCarthy about his contacts with Jordan comes days after Jordan, another of Trump’s top allies on Capitol Hill, suggested that he would ignore a request for an interview he received from the panel in December.

Thompson said that the committee was also seeking details about McCarthy’s conversations with Trump and Trump’s former White House chief of staff Mark Meadows before 6 January, suggesting an inquiry into what McCarthy knew of plans to stop Biden’s certification.

“We also must learn about how the president’s plans for January 6 came together,” Thompson said in the letter. “You reportedly explained to Mark Meadows and the former president that objections to the certification of the electoral votes on January 6 ‘was doomed to fail.’”

Having already established that McCarthy had informed Trump and Meadows before 6 January that the plan to stop Biden’s certification would not work, investigators want to learn why they were still “so confident the election result would be overturned”, Thompson said.

The Guardian first reported last week that the committee has in its possession messages turned over by Meadows and others suggesting the Trump White House coordinated with Republican lawmakers to stop Biden’s certification, according to sources familiar with the matter.

Thompson said that the committee was also interested in McCarthy’s communications with Trump in the week after the Capitol attack, including the possibility that Trump could have faced a censure resolution, impeachment and removal under the 25th amendment.

Thompson added that the panel was not interested in McCarthy’s political conversations with Trump when he visited the former president at Mar-a-Lago on 28 January, but was taking an interest in why his characterization of Trump’s culpability changed so dramatically.

“Did President Trump or his representatives discuss or suggest what you should say publicly, during the impeachment trial (if called as a witness), or in any later investigation about your conversations with him on January 6?” Thompson said in the letter.

The chairman also revealed for the first time that the select committee has contemporaneous messages showing McCarthy talked to Trump about his immediate resignation, among a number of other potential consequences he may have faced for inciting the Capitol attack.

“A full and accurate accounting of what happened on January 6th is critical to the select committee’s legislative recommendations. And the American people deserve to understand all the relevant details,” Thompson said, suggesting an interview in the first week of February.

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