As the House select committee investigating the January 6 Capitol attack was negotiating with Donald Trump’s former strategist Steve Bannon to cooperate with its inquiry, the panel affirmed one of their rules: no third-party lawyers could attend witness depositions.

That meant when Bannon’s then-attorney asked whether a lawyer for Trump could be present for the closed-door interview to decide what issues were covered by the former president’s invocation of executive privilege, the select committee flatly refused.

Now, that refusal appears set to feature as one of Bannon’s central arguments to defend against his contempt of Congress indictment that came after he entirely skipped his deposition last October and refused to produce documents as required by his subpoena.

The former Trump aide is advancing a high-stakes – and arcane-sounding – defense as he battles the justice department (DoJ) in a case that could mean up to a year in federal prison and thousands of dollars in fines if convicted – but potentially defang congressional power should he prevail.

The all-or-nothing nature of the defense is characteristic of Bannon, a fierce defender and confidante of the former president even after he departed the White House seven months into the Trump administration after a turbulent tenure as his chief strategist.

It was precisely because of his contacts with Trump in the days before January 6 that the select committee made Bannon one of the panel’s first subpoena targets as it seeks to uncover whether Trump oversaw a criminal conspiracy that culminated in the Capitol attack.

The crux of Bannon’s argument is that he could reasonably believe the subpoena was invalid when the select committee refused to allow a Trump lawyer to attend the deposition, after the former president asserted executive privilege over the materials covered by the subpoena.

The argument rests on a 2019 justice department office of legal counsel opinion (OLC) that says congressional subpoenas that prevent executive branch counsel from accompanying executive branch employees to depositions are “legally invalid” and not enforceable.

Bannon’s attorneys say the doctrine at issue is protecting the president’s constitutional authority to limit the disclosure of privileged information, which typically involves discussions with close presidential aides who need to be able to offer candid advice.

In that sense, the principle extends to Bannon, they argue: the supreme court decided in Nixon v GSA 1977 that former presidents could “assert” executive privilege, while a 2007 OLC opinion found executive privilege could cover discussions with private, non-executive branch employee advisers.

And since the select committee issued only one subpoena for both documents and testimony, when the subpoena was invalidated by the panel’s refusal to allow a Trump lawyer to attend, Bannon’s attorneys contend the document request element of the subpoena also became void.

“We don’t read OLC opinions in isolation,” David Schoen, one of three attorneys defending Bannon in this case – the others are Evan Corcoran and Bob Costello – told the Guardian in a text message. “They build on each other.”

The justice department does not think the OLC opinions protect Bannon, in part because he was not an executive branch employee at the time of January 6, and the select committee contends Trump did not formally assert executive privilege over subpoenaed materials.

The assistant US attorney Amanda Vaughn also indicated in recent court filings that Bannon’s argument – that the subpoena was invalid because House deposition rules excluded third-party lawyers – was in bad faith since his then-attorney, Bob Costello, never raised it as an issue at the time.

“Costello inquired – but said that he did not need an immediate answer – whether there was a way for a lawyer for President Trump to appear at the defendant’s deposition,” Vaughn said in the justice department’s response to Bannon.

But Bannon won an initial victory on the dispute last week after the judge in the case ordered the justice department to turn over OLC “writings” about its position on prosecuting current or former US officials claiming immunity from congressional subpoena over executive privilege.

US district judge Carl Nichols granted the request by Bannon’s attorneys, who suggest Costello was relying on the OLC opinions to current and former White House aides when he advised Bannon not to appear for his deposition since the select committee subpoena was invalid.

A spokesman for the select committee and a spokesman for the US attorney for the District of Columbia declined to comment.

The defense that Bannon is advancing – using broad readings of parts of the justice department’s own positions and amalgamating them into a wider argument – is controversial, but it underscores the complexities facing the justice department in pursuing the case.

“Bannon’s trying to use the OLC opinions as a shield that doesn’t quite cover him, but gives him enough of a defense to fend off the DoJ’s necessity of proving criminal intent,” said Jonathan Shaub, a law professor at the University of Kentucky and a former OLC attorney-adviser.

To establish criminal contempt of Congress, the justice department has to prove Bannon’s subpoena defiance was “willful” – which Bannon’s attorneys say should be interpreted as meaning whether the former Trump aide knew his conduct was unlawful or wrong.

The combined force of Bannon’s arguments, his attorneys say, demonstrate that he did not know his defiance was unlawful, and shifts the burden onto the justice department to prove beyond a reasonable doubt that Bannon did not believe the privilege claims were valid.

The former Trump aide is also coupling the argument that the subpoena was invalid with the defenses that he relied on the advice of counsel when he defied the subpoena, and that he cannot be prosecuted because OLC opinions – considered binding on the justice department – forbid it.

Bannon’s attorneys say the former Trump aide ignored his subpoena entirely on the advice of Costello, who relied on the OLC opinions about immunity for former presidential aides and was the only point of contact with the select committee during negotiations about his cooperation.

It is far from clear whether Bannon will prevail.

In a hearing last week, Nichols cast doubt on the advice of counsel argument, noting that the relevant case law, Licavoli v United States 1961, holds that relying on legal advice to defy a subpoena is no defense.

The justice department argued in their brief that the interpretation of “willful” should in fact remain the standard established by the Licavoli court: “a deliberate and intentional failure to appear or produce records as required” by a congressional subpoena.

But Bannon’s lawyers have noted that the Licavoli case did not involve executive privilege and therefore does not apply to Bannon, not least because the justice department has itself maintained that executive privilege cases are unique because of the constitutional implications.

“He’s reaching for all the straws that he can,” Shaub said of Bannon. “It may succeed or it may not. But if Nichols rules against him, he’s certainly going to take it to the DC circuit or even up to the supreme court. He’s definitely playing a longer game here.”