House impeachment manager Rep. Adam Schiff (D-CA) said Monday that “you can’t have a meaningful trial” without hearing from former National Security Advisor John Bolton,. However, in September, 2006 Schiff penned a letter to the Senate Foreign Relations Committee in opposition to his nomination as U.S. Ambassador to the United Nations.

The star witness in Schiff’s trial was once his national security nightmare. In the letter, Schiff blasted Bolton saying his “conduct and belligerent style alienated key allies and proved ineffective, most visibly in the failure to make any progress on consolidating UN operations to make them more efficient and cost-effective.”

“At a time when the United States and other Security Council members must work together to block Iran’s quest for the nuclear fuel cycle and build a lasting peace in Lebanon, we need a representative at the UN who is deeply committed to the multilateral diplomacy needed to effectuate a successful resolution of these crises,” Schiff wrote.

Some critics suggest Ambassador Bolton has had a long record of hostility to working with other international actors. Bolton has also expressed skepticism about whether the UN should have any role in peacekeeping efforts.”

The New York Times reported Sunday several excerpts from Bolton’s upcoming tell-all book where he is said to recount his White House tenure. In the book, Bolton says that the President’s hold on aid to Ukraine “was separate” from the investigations into the Bidens, according to the report. The timing of the leaks was curious as the impeachment trial continues and the Senate weighs the option of calling Bolton to testify as a witness.