WASHINGTONWASHINGTON (Reuters) - A U.S. judge expressed skepticism on Thursday toward a bid by President Donald Trump's former campaign chairman to get a raft of charges dismissed, while a prosecutor said Paul Manafort was suspected of being a campaign "back channel" to Russia.

During a hearing, U.S. District Court Judge Amy Berman Jackson questioned Manafort's attorney Kevin Downing's assertion that Special Counsel Robert Mueller overstepped his authority in bringing charges that Downing said fell outside of parameters set by the Justice Department's No. 2 official Rod Rosenstein.

Jackson signaled some willingness to consider whether at least one count in the indictment could be dismissed because the criminal action cited was covered by another charge.

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Manafort faces indictments by Mueller in Washington and Virginia, accusing him of conspiracy against the United States and other crimes.

The hearing was the latest development in a special counsel investigation that could threaten Trump's presidency, looking into potential collusion between the president's 2016 campaign and Moscow. Downing asked Jackson to dismiss the Washington indictment.

"Here you have somebody who was a campaign official in the Trump campaign, where he had longstanding ties to Russia-backed politicians in the Ukraine," Michael Dreeben, a prosecutor on Mueller's team, said of Manafort. "What (was) the nature of those connections? Did they provide a means for surreptitious communications? Did they provide a back channel to Russia?"

Downing argued that Rosenstein's order appointing Mueller in May 2017 was overly broad and violated Justice Department rules on the naming of special counsels, and that Mueller exceeded even the wide latitude Rosenstein provided.

Jackson expressed doubts over arguments that Mueller should not have been able to bring charges Downing called unrelated to Russia or Moscow's potential coordination with Trump's campaign.

While Manafort has pleaded not guilty, Mueller has extracted guilty pleas from other former campaign figures including Manafort's former deputy and business partner Rick Gates.

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Downing said the FBI in 2014 closed out an earlier investigation of Manafort's Ukraine business dealings without pursuing charges, meaning the matter cannot be covered by Mueller's probe.

Dreeben said the special counsel had explicit authority from Rosenstein to probe Manafort's Ukraine dealings based on an August 2017 memo.

"It is not a blank check," Dreeben said of Rosenstein's parameters for the probe.

The memo was written a month after the FBI executed a search warrant at Manafort's home in Alexandria, Virginia. Downing said Rosenstein's memo seemed to come "after the fact" to justify the investigation.