Former FBI Director James Comey is sworn in prior to testifying before a Senate Intelligence Committee hearing on Russia's alleged interference in the 2016 U.S. presidential election on Capitol Hill in Washington, U.S., June 8, 2017. REUTERS/Jonathan Ernst

By Sarah N. Lynch and Andrew Goudsward

ALEXANDRIA, Virginia (Reuters) -Former FBI Director James Comey and New York Attorney General Letitia James will ask a federal judge on Thursday to drop the criminal charges against them, arguing that President Donald Trump's hand-picked U.S. attorney, who obtained the indictments against them, was unlawfully appointed.

The hearing at the federal courthouse in Alexandria, Virginia, will mark the first time a judge will consider one of several efforts James and Comey have made to dismiss the indictments before trials.

The arguments center on whether Lindsey Halligan, Trump's former personal attorney, was illegally installed as interim U.S. attorney for the Eastern District of Virginia. If Halligan is deemed to be unlawfully appointed, then the charges against Comey and James could be invalidated since Halligan was the only federal prosecutor to present evidence to the grand juries in both matters.

Comey has pleaded not guilty to charges of making false statements and obstructing Congress, while James has pleaded not guilty to charges of bank fraud and lying to a financial institution. Both were charged by Halligan's office shortly after Trump openly called on Attorney General Pam Bondi to prosecute them.

Bondi appointed Halligan at Trump's request in September, after her predecessor, Erik Siebert, was forced out of the job after expressing concerns about a lack of evidence to support criminal charges against both Comey and James, two prominent critics of Trump who each oversaw investigations into him.

COMEY, JAMES TO ARGUE APPOINTMENT VIOLATES LENGTH LIMIT

Attorneys for Comey and James will argue that Halligan's appointment violates a federal law they said limits the appointment of an interim U.S. attorney to one 120-day stint. Repeated interim appointments would bypass the Senate confirmation process and allow a prosecutor to serve indefinitely, they said.

Siebert had been previously appointed by Bondi for 120 days and was then appointed by the U.S. District Court for the Eastern District of Virginia, since the Senate had not yet confirmed him in the role.

Thursday's arguments will be heard by Cameron McGowan Currie, a South Carolina-based federal judge appointed by former Democratic President Bill Clinton, who was assigned to decide the issue given the role federal judges in Virginia played in Siebert's appointment.

The Justice Department plans to argue that Halligan's appointment was lawful, saying that nothing in the law "explicitly or implicitly precludes the Attorney General from making additional appointments."

In an effort to bolster its case, Bondi also belatedly, in late October, separately gave Halligan a second title of "Special Attorney" and said she is authorized to supervise both prosecutions.

Outside legal experts, however, have said the Justice Department's unusual maneuvers to install Halligan could derail the cases.

Three federal judges in other cases have already separately ruled against the Justice Department on the issue, finding that Bondi unlawfully appointed U.S. attorneys in New Jersey, Nevada and Los Angeles.

In addition, a Justice Department memo penned in 1986 by Samuel Alito, who is now one of the Supreme Court's conservative justices, interprets the law the same way Comey and James do.

(Reporting by Sarah N. Lynch and Andrew GoudswardEditing by Rod Nickel)