WASHINGTON (Reuters) - President Donald Trump’s longtime political ally Roger Stone pleaded not guilty on Tuesday to charges that he tried to obstruct a congressional investigation into allegations that Russia meddled in the 2016 presidential election.

Stone, a self-proclaimed “dirty trickster” and Republican political operative for decades, pleaded not guilty in a federal court in Washington, D.C., to lying to Congress, obstructing an official proceeding and witness tampering.

He is the latest member of Trump’s inner circle charged in Special Counsel Robert Mueller’s Russia investigation and could face about 50 years in prison if found guilty on all the charges, although he is unlikely to receive such a harsh sentence, sentencing experts say.

Prosecutors say Stone, 66, lied to investigators for the House of Representatives Intelligence Committee who were looking into allegations that Russia hacked the emails of senior Democrats.

The indictment against Stone also says he told members of Trump’s 2016 presidential campaign that he had advance knowledge of plans by the WikiLeaks website to release damaging emails about Trump’s Democratic opponent in 2016, Hillary Clinton. U.S. intelligence agencies say the emails were stolen by Russia.

Stone, a Republican operative since the days of the Watergate scandal that forced President Richard Nixon to resign in 1974, has been a friend and ally of Trump for some 40 years.

Usually exuberant, Stone clenched his jaws as he sat quietly waiting for the judge in the courtroom. He then responded to the judge’s questions with courteous, clipped answers.

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Outside after the hearing, Stone flashed the twin “V for Victory” signs that Nixon was famous for but did not address the media.

A small group of protesters waved Russian flags and a placard that said “Dirty Traitor” while his supporters called for Mueller to be fired.

The indictment did not indicate whether Stone knew that Russians had stolen the emails by hacking into computers used by Clinton’s senior campaign adviser John Podesta and the Democratic National Committee.

The charges against Stone marked the first time Mueller’s team has publicly tied the Trump campaign to WikiLeaks, and raised questions about what Trump may have known prior to the public release of the stolen emails.

Stone, who is free on a $250,000 bond after being arrested at his Florida home last week, has accused Mueller of “a raw abuse of power.”

Trump has called the investigation a witch hunt and denied collusion. Russia has denied U.S. intelligence community’s finding that Moscow interfered in the U.S. political arena.

Thirty-four people have been swept up in the Mueller investigation. Those charged include Trump’s former campaign chairman and deputy campaign chairman, former national security adviser and his former personal lawyer.