Instead of backing down, have the rebels of the Whiskey Rebellion use guerilla warfare against Washington and his men. While marching through Western PA, have Washington get struck down by a marksman.
As for Madison, fight to the death in the White House during the War of 1812 like previously said.
Abe Lincoln visits Grant during the siege of Petersburg to discuss the war, Confederates just so happen to launch a massive sortie from their trenches at the same time, they reach Grant's headquarters, a fight ensues, and Lincoln is shot.
Teddy Roosevelt personally leads an army to Panama to fight the Colombians. Is wounded in battle, gets gang green as well as some south American disease. Immune system can't handle it and Teddy dies a miserable death one week later.
Terrorists assault the White House. Although it's overall a massive failure, an RPG hit the West Wing before Bush had the chance to make it to safety. He died instantly. ALL HAIL PRESIDENT CHENEY!