In 1789, the Founding Fathers came up with a system of checks and balances to keep kingly powers out of the hands of American presidents. But in the 1970s and '80s, a faction of Republican loyalists, outraged by the fall of the imperial presidency after Watergate and the Vietnam War, abandoned conservatives' traditional suspicion of concentrated government power. These men hatched a plot that would allow the White House to return to, or even surpass,...