Washington DC - Prosecutors overstepped in charging January 6 rioters with obstruction for trying to prevent certification of the 2020 presidential election, the US Supreme Court said Friday in a case that could see dozens of convictions overturned.

The matter was brought to the court in the case of a former police officer Joseph Fischer, a supporter of former president Donald Trump who entered the Capitol in Washington, DC with hundreds of others on January 6, 2021.

Writing the opinion for the majority, Chief Justice John Roberts said prosecutors' interpretation of the law would "criminalize a broad swath of prosaic conduct, exposing activists and lobbyist(s) to decades in prison."

The government "must establish that the defendant impaired the availability or integrity for use in an official proceeding of records, documents, objects, or other things used in an official proceeding, or attempted to do so," he wrote.

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The case was decided 6-3, with Ketanji Brown Jackson joining with the court's conservatives. Amy Coney Barrett, appointed by Trump, penned the dissent, which was joined by liberal judges Sonia Sotomayor and Elena Kagan.

It now heads back to a lower court, which will decide whether Fischer's indictment can still stand in light of the narrower interpretation of "obstruction."

In all, 52 rioters have been convicted and sentenced on obstruction charges, with 27 currently incarcerated, the Department of Justice said in a statement.

"The vast majority of the more than 1,400 defendants charged for their illegal actions on January 6 will not be affected by this decision," said Attorney General Merrick Garland.

At the core of the case was how to interpret the word "otherwise" in the relevant statute, the Sarbanes-Oxley Act, which was enacted after the destruction of documents in the 2001 Enron scandal.

This imposes up to 20 years in prison for whoever corruptly tampers with documents in an attempt to prevent them from being used in official proceedings, or "otherwise obstructs, influences, or impedes any official proceeding, or attempts to do so."

The word "otherwise" grants prosecutors excessive latitude, allowing them to pursue charges that far exceed the scope of Congress's initial purpose, the majority said.

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But in her dissent, Barrett said the fact that the joint session by Congress on January 6 was an official proceeding was not in dispute.

"Given these premises, the case that Fischer can be tried for 'obstructing, influencing, or impeding an official proceeding' seems open and shut," she wrote, accusing the majority of performing "textual backflips to find some way – any way" to narrow the reach of the relevant subsection.

This case also has potentially significant implications for Donald Trump, who faces four felony charges brought by Special Counsel Jack Smith over his alleged efforts to overturn the results of the 2020 election.

The Republican presidential candidate is charged with conspiracy to defraud the United States, conspiracy to obstruct an official proceeding, and obstruction of an official proceeding – the session of Congress held to certify Biden's victory.

He is also charged with conspiracy to deny Americans the right to vote and to have their votes counted.

But this case is on hold until the Supreme Court rules on Trump's claims that he is immune from criminal prosecution, which the judges are now expected to deliver on Monday.