WASHINGTON — President Donald Trump's former campaign chairman is suing special counsel Robert Mueller and the Justice Department.

Paul Manafort says prosecutors overstepped their bounds by charging him for conduct that he says is unrelated to Russian interference in the 2016 presidential election.

The lawsuit is the most direct challenge to date to Mueller's legal authority and the scope of his mandate as special counsel.

It comes amid Republican allegations of partisan bias among members of Mueller's team, which for months has been investigating whether the Trump campaign coordinated with Russia to influence the outcome of the U.S. election.

A Justice Department spokesman said: "The lawsuit is frivolous, but the defendant is entitled to file whatever he wants."