There always has been an element of criminality in American politics, and not simply because Richard Nixon was chased from office for his sins, which were egregious. His 1960 rival, John F. Kennedy, won the White House under suspicious circumstances, and Mr. Kennedy’s successor, Lyndon Johnson, won his Senate seat (and his riches) under conditions Common Cause, and most of the rest of us, would find repugnant today.

Until Watergate, those irregularities were the subject of insiders’ whispers, seldom piercing the public consciousness, perhaps because over time they roughly canceled each other out, Republican sin for Democratic transgression. But the politics above the surface always had discernible, understandable and dependable rules, and many of them are gone now.

Every American election since 1932, with the exception of 1960, has provided a choice between a liberal candidate and a conservative opponent. This arrangement has organized our politics even when the two parties had both conservative and liberal wings. (Those outlier wings — conservatives for the Democrats and liberals for the Republicans — began to weaken in the 1960s, were threatened in the Reagan 1980s and disappeared with the new century.) No one had any doubt which candidate, George W. Bush or Al Gore, was the conservative in 2000, nor was there any debate about the ideologies of Barack Obama and Mitt Romney a dozen years later.