The lawyer for President Trump’s former national security adviser John Bolton and his deputy, Charles Kupperman, said the two will not participate in the House impeachment inquiry until a federal judge resolves the specific dispute over whether they are legally permitted to testify.

“It is important both to Dr. Kupperman and to Ambassador Bolton to get a definitive judgment from the Judicial Branch determining their Constitutional duty in the face of conflicting demands of the Legislative and Executive Branches,” attorney Charles Cooper wrote in a letter to House lawyers released Friday.

The letter came in response to House Democrats’ request this week that Kupperman drop his novel lawsuit seeking to resolve conflicting orders over his participation in the impeachment inquiry into Trump.

House Democrats withdrew Kupperman’s subpoena this week, and, in the interest of speed, said lawmakers would instead be guided by the outcome of a separate case involving a subpoena for testimony from former White House counsel Donald McGahn. That case is further along in judicial proceedings and raises similar questions about the limits of immunity from congressional subpoena for high-ranking White House officials.

But Kupperman’s lawyer said in his letter Friday that the issues are distinct and require a separate ruling on the White House’s claims of “absolute testimonial immunity” for top aides. Unlike McGahn, Cooper emphasized that Bolton and Kupperman participated in meetings and conversations involving sensitive national security discussions at the heart of the impeachment inquiry.

Cooper told House leaders that Bolton, in particular, was also involved in “many relevant meetings and conversations that have not yet been discussed in the testimonies thus far.”

Both Kupperman and Bolton are prepared to testify, he concluded, “if the Judiciary resolves the conflict in favor of the Legislative Branch’s position respecting such testimony,” Cooper wrote.

“If the House chooses not to pursue through subpoena the testimony of Dr. Kupperman and Ambassador Bolton, let the record be clear: that is the House’s decision.”