Judge skewers Manafort's case challenging Mueller's powers

WASHINGTON, April 4 (Reuters) - A federal judge tore into all of the legal arguments that a lawyer for President Donald Trump's former campaign chairman Paul Manafort made on Wednesday in his long-shot civil case to convince her that Special Counsel Robert Mueller's investigation has run amok and should be reined in. "I don't really understand what is left of your case," U.S. District Court Judge Amy Berman Jackson said to Kevin Downing, Manafort's attorney, after peppering him with a lengthy series of questions.

Manafort filed a civil lawsuit on Jan. 3 against Mueller and Deputy Attorney General Rod Rosenstein, the Justice Department official who appointed the special counsel, in a key legal test of how far Mueller's mandate extends.

Mueller is investigating potential collusion between Trump's campaign and Russia as well as whether the president has unlawfully tried to obstruct the probe. Manafort, who performed lobbying work for a pro-Russian former Ukrainian president before serving as Trump's campaign chairman in the 2016 U.S. election, is facing two indictments brought by Mueller in federal courts in Washington and Alexandria, Virginia. The charges against him include conspiring to launder money, failing to register as a foreign agent, bank fraud and filing false tax returns.

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Manafort has pleaded not guilty and none of the charges directly relate to work he performed for Trump's campaign.

Manafort's civil lawsuit relies on an arcane law called the Administrative Procedure Act, which spells out the process federal agencies must follow when writing regulations. It alleges portions of Rosenstein's order laying out Mueller's investigative mandate violate Justice Department rules.

(Reporting by Sarah N. Lynch; Editing by Will Dunham and Bill Trott)