Rudy Giuliani, who served as Donald Trump’s hard-charging attorney during failed legal efforts to overturn his presidential election defeat, was accused of “weaponizing” his law license, during a turbulent first hearing of a disciplinary panel in Washington DC mulling ethics charges against him.

The former New York mayor became argumentative on Monday as Hamilton Fox, of the DC disciplinary counsel’s office, suggested Giuliani had insulted the US constitution with a series of “frivolous” and ultimately futile lawsuits on the former president’s behalf.

Giuliani is facing charges of violating attorney conduct rules in what is expected to be a two-week hearing before the Washington DC board of professional responsibility. It is seen as an important first case that could lead to further steps against Trump’s team of election-result denying lawyers.

“A constitutional democracy like ours does not work unless the loser honors the decision of the voters,” Fox said, referring to a blitz of post-election litigation that Trump tapped Giuliani to lead after he lost the 2020 election to Joe Biden.

In at least one state, Fox said, Giuliani appeared personally, having put together the band of lawyers at short notice to try to keep the outgoing president in office.

“Mr Giuliani was responsible for filing a frivolous action asking a court in Pennsylvania to deny millions of people the right to vote,” Fox said.

The Pennsylvania lawsuit led to a bizarre and much mocked press conference that Giuliani hosted at the Four Seasons Landscaping company in Philadelphia. The venue, next to a sex shop, was close to the glitzy Four Seasons hotel that many assumed Giuliani had intended to book.

Giuliani grew visibly angry and evasive during his testimony on Monday, leading to rebukes from board chair, Robert Bernius. He responded “not that day” to a question whether he had co-authored the Pennsylvania lawsuit.

“I’m asking you what time it is, and you’re telling me how to make a watch,” Fox said.

The exchange prompted an intervention from Bernius, who reminded Giuliani that his years of legal experience would make him aware he was required to answer questions honestly as a witness.

Giuliani was charged in June with ethical misconduct, and lost his license to practice law in Washington DC, one year after his New York license was suspended as a state appeals court found he made “demonstrably false and misleading” statements that widespread voter fraud undermined the election.

Giuliani has not been charged with any criminal misconduct in relation to election lawsuits.

He also learned last month that he would not face charges after a federal investigation into his lobbying activities on Trump’s behalf in Ukraine. The FBI raided Giuliani’s New York apartment and office in May 2021, seizing multiple electronic devices.

During Monday’s first session, Giuliani said: “I believe that I’ve been persecuted for three or four years, including false charges brought against me by the federal government,” he said.

On the Pennsylvania lawsuit, Fox said that Giuliani wanted a federal judge to throw out almost 700,000 mail-in ballots, and had suggested that 7m ballots could ultimately be invalidated. But he noted no specific allegation of fraud was presented to that court, nor any proof offered that it had occurred.

The lawsuit failed in district court, and an appeals court refused to hear a revised complaint.