Federalist Society then-Executive Vice President Leonard Leo speaks to media at Trump Tower in New York, on November 16, 2016.
CNN  — 

Senate Democrats on Thursday issued a subpoena to a conservative legal advocate they are investigating in response to a series of ethics controversies at the Supreme Court involving lavish travel and gifts to justices.

Months after voting to authorize the subpoena, the Senate Judiciary Committee formally issued one to Leonard Leo, committee chairman Dick Durbin said. Democrats say the subpoena is necessary to better understand whether specific individuals and groups have used undisclosed gifts to gain access to the justices.

“Mr. Leo has played a central role in the ethics crisis plaguing the Supreme Court and, unlike the other recipients of information requests in this matter, he has done nothing but stonewall the committee,” Durbin said in a statement to CNN. “This subpoena is a direct result of Mr. Leo’s own actions and choices.”

Leo, a prominent conservative legal advocate and board co-chairman of the influential Federalist Society, confirmed Thursday he had been subpoenaed and vowed to not comply. Calling the subpoena “unlawful” and “politically motivated,” Leo said in a statement to CNN that he was “not capitulating” to what he described as the “left’s dark money effort to silence and cancel political opposition.”

Leo’s attorney, David Rivkin, sent a letter to Durbin asserting that he is “not complying” with the “unlawful and politically motivated subpoena.”

The committee voted along party lines in November to authorize subpoenas for Leo and GOP megadonor Harlan Crow in response to revelations of travel accepted by several Supreme Court justices, including Justices Clarence Thomas and Samuel Alito.

Crow paid for lavish trips for Thomas, which initially went unreported on the justice’s financial disclosure reports, according to a series of reports in ProPublica. Leo arranged a 2008 fishing trip attended by Alito, according to a ProPublica report. Alito similarly did not report the trip on his financial forms.

It’s not clear why Durbin, an Illinois Democrat, waited months between the vote to authorize the subpoenas and actually issuing one.

The committee vote last fall broke down in partisan rancor as Republicans accused Democrats of playing politics and walked out of the hearing. If Leo ultimately does not comply, Democrats could be forced to hold a vote and find 60 votes in the split chamber to enforce it.