Mr. Dershowitz told Mr. Nader’s allies that he had reached out to an official in the Trump administration and one in the Israeli government to try to assess whether they would support a plan for Mr. Nader to be freed from United States custody in order to resume a behind-the-scenes role in Middle East peace talks, and whether Mr. Trump might consider commuting his 10-year sentence.

Mr. Dershowitz helped craft a proposal — which Mr. Nader’s allies believed he was floating at the White House in the final days of the Trump presidency — for Mr. Nader to immediately “self-deport” after his release from a Virginia jail. Under the plan, Mr. Nader would board a private plane provided by the United Arab Emirates to return to the Gulf state, where he holds citizenship and has served as a close adviser to the powerful crown prince.

Given the nature of Mr. Nader’s crimes and his cooperation with the Russia investigation, his bid for clemency was a long shot that did not work out. But Mr. Dershowitz’s willingness to pull a range of levers to try to free him shows why he emerged as a highly sought-after and often influential intermediary as Mr. Trump decided who would benefit from his pardon powers.