A federal judge has refused to dismiss an indictment charging four alleged Proud Boys leaders with conspiracy and obstructing an official proceeding during the 6 January attack on the US Capitol – a development that could have potential implications for Donald Trump.

On Tuesday, the US district judge Timothy Kelly rejected arguments by defence lawyers that Ethan Nordean, Joseph Biggs, Zachary Rehl and Charles Donohoe are charged with conduct that is protected by the first amendment right to free speech.

He said they had multiple nonviolent options to express their opinions about the 2020 presidential election, in which former president Trump lost to Joe Biden.

“Defendants are not, as they argue, charged with anything like burning flags, wearing black armbands, or participating in mere sit-ins or protests,” he wrote in a 43-page ruling. “Moreover, even if the charged conduct had some expressive aspect, it lost whatever first amendment protection it may have had.”

As reported by the Guardian last week, expectation is growing that Trump may face charges for trying to obstruct Congress from certifying Biden’s election this year as a House panel is collecting more evidence into the attack.

Evidence against the former president could include his involvement in the “Stop the Steal” rally near the White House where he called on supporters to march to the Capitol and “fight like hell [or] you’re not going to have a country any more”.

Federal prosecutors have so far cited the obstruction statute in about 200 cases involving rioters charged by the Department of Justice for their involvement in the attack, supported by recent rulings by Trump-appointed district court judges.

Nordean, Biggs, Rehl and Donohoe, who are jailed while they await a trial scheduled for May, were indicted in March on charges including conspiracy and obstructing an official proceeding.

Nordean, from Auburn, Washington, was a Proud Boys chapter president and member of its national “elders council”. Biggs, from Ormond Beach, Florida, is a self-described Proud Boys organiser. Rehl was president of the group’s chapter in Philadelphia and Donohoe, from Kernersville, North Carolina, was president of his local chapter, according to the indictment.

Defence lawyers also argued the obstruction charge was not applicable to their clients’ cases because they claimed the certification of the electoral college by Congress was not an “official proceeding”, but Kelly disagreed.

Earlier this month another judge in the District of Columbia’s federal court upheld prosecutors’ use of the same obstruction charge in a separate case against riot defenders.

To date more than 700 people have been charged with federal crimes related to the Capitol riot and at least 165 of them have pleaded guilty.