Former Trump aide Steve Bannon believed he was "above the law" when he defied a subpoena to testify before lawmakers investigating the 2021 Capitol assault, a US prosecutor said Tuesday as arguments began in his federal trial for contempt of Congress.

Bannon, who led Donald Trump's successful 2016 presidential election campaign, was among dozens of people called by a House of Representatives committee to testify about the storming of Congress by Trump supporters on January 6, 2021.

The 68-year-old did not appear on the summons date or provide requested documents related to the attack and events leading up to it, and was indicted on two charges of contempt.

"The defendant decided he was above the law," said prosecutor Amanda Vaughn in her opening statement to the panel of 12 jurors and two alternates selected on Tuesday at a courthouse in Washington. "And that's why we're here today."

"He ignored orders to comply, even after Congress had rejected his excuses not to, and he put aside multiple warnings that he could face criminal prosecution," she said.

Bannon's lawyer Evan Corcoran denied that his client had ignored the subpoena, saying the date was "the subject of ongoing discussions and negotiation" and "flexible," noting this was typical procedure for the House committee.

According to the panel, Bannon spoke to Trump the day before thousands of the former president's supporters stormed the Capitol in an effort to block the certification of Democrat Joe Biden's election victory.

They had been egged on by Trump in a fiery speech near the White House, during which he repeated his false claims of election fraud.

Corcoran said the decision to hold Bannon in contempt was politically motivated, saying "politics is the lifeblood of the US House of Representatives" and it "invades every decision that they make."

Vaughn said the committee had cause to believe Bannon and other Trump advisors could have information on links between the White House and the rioters.

After refusing to testify for months, Bannon finally agreed this month to cooperate with the House investigation, a move prosecutors said previously was a "last-ditch attempt to avoid accountability."

Bannon's lawyers had sought to delay the start of the trial but US District Court Judge Carl Nichols denied the motion.

Prosecutors called their first witness, committee deputy staff director and chief counsel Kristin Amerling, near the end of the session.

Bannon, who served as Trump's strategy chief at the White House before being sacked in 2017, could serve 30 days to a year in jail for each of the two counts if found guilty in what is expected to be a speedy trial.