How about Powers Boothe?
Others have correctly pointed to other fine performances in this film, and I have to throw Boothe, as Alexander Haig, into the mix. I lived through, and followed, the Nixon years and the Reagan assassination attempt - after which Haig announced he was in control at the White House, though a guy named George Bush was wandering around somewhere. From what I've read, Boothe was pretty darn good at portraying the steely resolve of General Haig.
BTW, if many accounts, including Woodward and Bernstein, are correct, Gen. Haig may have held the government together when it could have descended into chaos. Haig may have actually been a hero.