The House select committee investigating the January 6 Capitol attack issued a subpoena on Wednesday to former Trump White House counsel Pat Cipollone, compelling him to testify about at least three parts of Donald Trump’s efforts to overturn the 2020 election results.

The subpoena marked a dramatic escalation for the panel and showed its resolve in seeking to obtain inside information about how the former president sought to return himself to office from the unique perspective of the White House counsel’s office.

“Mr Cipollone repeatedly raised legal and other concerns about President Trump’s activities on January 6th and in the days that preceded,” the chairman of the select committee, Bennie Thompson, said in a statement accompanying the subpoena.

“The committee needs to hear from him on the record, as other former White House counsels have done in other congressional investigations. Concerns Mr Cipollone has about the prerogatives of the office he previously held are clearly outweighed by the need for his testimony.”

Cipollone was a key witness to some of Trump’s most brazen schemes to overturn the 2020 election results, which, the select committee has said in its hearings, was part of a sprawling and potentially unlawful multi-pronged strategy that culminated in the Capitol attack.

Cipollone has information about Trump’s push to send fake slates of electors to Congress, the subpoena letter said, a plot that would have given then-vice-president Mike Pence cover to supposedly refuse to certify Joe Biden’s election win.

He also has information about Trump’s foiled plan to pressure the justice department into falsely declaring the results of the 2020 election “corrupt”, the subpoena letter said, and potentially illegal conduct on the part of the former president on 6 January.

Former White House aide Cassidy Hutchinson, according to her public testimony, was told by Cipollone that “we’re going to be charged with every crime imaginable” if Trump went to the Capitol that day as he pressured Congress to not certify Biden’s win.

Thompson acknowledged in the subpoena letter that Cipollone had spoken to House investigators in a more informal setting on 13 April. But, he said, recent evidence to which he was in a “unique position” to discuss necessitated on-the-record testimony at a 6 July deposition.

The panel had been negotiating Cipollone’s testimony for weeks without success, with Cipollone apparently concerned about its scope. A spokesperson for Cipollone did not respond to requests for comment about whether he would comply with the subpoena or litigate.

Cipollone remained in the Trump administration through its final weeks and months, effectively becoming a fact witness to Trump’s thinking and conduct as the former president scrambled to find any way to keep himself in office after losing the election.

Together with his deputy, Pat Philbin, and another White House lawyer Eric Herschmann, who has cooperated extensively with the select committee, Cipollone sought to restrain some of Trump’s most dangerous impulses, fearing Trump could face serious legal exposure.

In doing so, former Trump White House aides say, Cipollone became one of the final senior administration officials who acted as a guardrail for Trump. After leaving the administration, Cipollone returned to private practice as a partner at Ellis George Cipollone LLP.

Should Cipollone testify to the select committee, he may seek advice from another lawyer at EGC: former Nixon deputy White House counsel Fred Fielding, who worked under Nixon White House counsel John Dean, and testified at the Watergate trial.