McMaster is a retired U.S. Army lieutenant general, who served as the 25 th United States National Security Advisor from 2017 to 2018, under Trump. He is currently a senior fellow at the Hoover Institution at Stanford University. Newsweek reached out to McMaster via email for further comment.

"I would appeal to his sense of, 'Don't be a loser,'" Rep. McCaul says he'd tell Trump regarding aid to Ukraine. "You don't want to lose this."

Trump has promised to end Russia's full-scale invasion of Ukraine within a day if elected but has not publicly elaborated on how he plans to achieve that.