WASHINGTON (Reuters) - The U.S. Supreme Court gave a boost on Friday to a Pennsylvania man who challenged an obstruction charge brought against him concerning the Jan. 6, 2021, attack on the Capitol in a ruling with potential implications for the federal criminal case against Donald Trump for trying to undo his 2020 election loss.

The justices ruled 6-3 to throw out a lower court's decision that had allowed a charge of corruptly obstructing an official proceeding - the congressional certification of President Joe Biden's victory over Trump that the rioters sought to prevent - against defendant Joseph Fischer, a former police officer. The justice directed the lower court to reconsider the matter.

Fischer had challenged the obstruction charge, which federal prosecutors brought against him and hundreds of others - including Trump - in Jan. 6-related cases. The ruling was a setback for the U.S. Justice Department and President Joe Biden's administration and a potential boost for Trump.

Trump, the Republican candidate challenging Democratic President Joe Biden in the Nov. 5 U.S. election, was hit with the obstruction charge as part of a four-count criminal indictment in a case brought last year by Special Counsel Jack Smith. The charge falls under the 2002 Sarbanes-Oxley Act, a federal law passed after the accounting fraud scandal at now-defunct energy company Enron.

Fischer was accused by prosecutors of charging at police guarding a Capitol entrance during the attack. Fischer, at the time a member of the North Cornwall Township police in Pennsylvania, got inside the building and pressed up against an officer's riot shield as police officers attempted to clear rioters, according to prosecutors. He remained in the Capitol for four minutes before police pushed him out, they said.

Fischer has been awaiting trial on six other criminal counts, including assaulting or impeding officers and civil disorder, while his challenge to his obstruction charge has proceeded.

U.S. District Judge Carl Nichols, a Trump appointee, granted Fischer's request to dismiss the obstruction charge, ruling it applies only to defendants who tampered with evidence. The U.S. Court of Appeals for the District of Columbia Circuit reversed that decision, prompting Fischer's appeal to the Supreme Court.

Federal prosecutors estimate that about 250 of the roughly 1,400 people charged in the Capitol attack by Trump supporters could be impacted by the ruling. According to Justice Department data, about 50 Jan. 6 defendants were convicted and sentenced on the obstruction charge with no other felony. Of those, about half are currently serving a sentence of incarceration - representing less than 2% of all charged cases.

The charge carries a sentence of up to 20 years in prison if convicted, though Jan. 6 defendants convicted of obstruction have received far lesser sentences.

The legal issue in the case involved how two parts of the obstruction law fit together. The first provision prohibits obstructing an official proceeding by destroying "a record, document or other object." The second part makes it a crime to "otherwise obstruct" an official proceeding.

The Justice Department argued that Congress included the second provision to give the obstruction law a broad sweep.

About two thirds of the Jan. 6 defendants charged with obstruction also were charged with other felonies.

The Supreme Court heard arguments in the case in April.

After the 2020 election, Trump and his allies made false claims that it had been stolen from him through widespread voting fraud. On the day when Congress met to certify Biden's victory, Trump supporters stormed the Capitol, broke through barricades, attacked police officers, vandalized the building and forced lawmakers and others to flee for safety.

In August 2023, Smith brought four federal criminal counts against Trump in the election subversion case: conspiring to defraud the United States, corruptly obstructing an official proceeding and conspiring to do so, and conspiring against the right of Americans to vote.

Trump in a separate case brought in New York state court was convicted by a jury in Manhattan on May 30 on 34 counts of falsifying documents to cover up hush money paid to a porn star to avoid a sex scandal ahead of the 2016 election.

Trump also faces election-related criminal charges in state court in Georgia. He has called all the cases against him politically motivated.

The Supreme Court heard arguments on April 25 on Trump's bid for immunity from prosecution for trying to undo his 2020 election loss. The court's conservative justices signaled support for U.S. presidents having some level of immunity from criminal charges for certain acts taken in office. A ruling in that case is expected in the coming days.

In another closely watched Trump-related case, the Supreme Court on March 4 overturned a judicial decision that had barred him from the ballot in Colorado office under a constitutional provision involving insurrection. Colorado's top court had found that Trump took part in an insurrection for inciting and supporting the Capitol attack.

Justice Samuel Alito rejected calls by some Democratic lawmakers to recuse himself from the obstruction and presidential immunity cases after revelations that two flags like those carried by the pro-Trump rioters during the Capitol attack had flown outside his Virginia home and New Jersey vacation home.

(Reporting by Andrew Chung; Editing by Will Dunham)

By John Kruzel