The brass-knuckle 1964 campaign is remembered for Lyndon B. Johnson’s alarmist “daisy ad,” which suggested that Barry M. Goldwater’s election might lead to nuclear war. But it rested on statements Goldwater had made indicating a loose attitude toward nuclear weapons. (“Lob one into the men’s room in the Kremlin,” he once joked.)

The attack ads devised by the strategist Lee Atwater for Vice President George Bush in the 1988 campaign, one of the dirtiest ever, were grounded in at least a kernel of truth. Mr. Bush’s opponent, Michael S. Dukakis, might not have deserved blame for the furlough program that let Willie Horton commit additional crimes, but at least the program and prisoner were real. Atwater exploited these events, but did not invent them.

At least four factors since the 1970s have lowered the cost for politicians who lie and, more important, repeat their fabrications through their attack ads. First is the overall decline in respect for institutions and professionals of all kinds, from scientists and lawyers to journalists and civil servants.

Second are changes in media regulation and ownership. In 1985, the conservative organization Fairness in Media, backed by Senator Jesse Helms, tried to arrange a takeover of CBS and “become Dan Rather’s boss.” It failed, but two years later conservatives set the stage for an even bigger triumph. For decades, radio and television broadcasters had been required to present multiple viewpoints on contentious public debates on the grounds that they were stewards of the public airwaves. But in 1987, members appointed by President Ronald Reagan to the Federal Communications Commission abolished this “fairness doctrine.” The change facilitated the creation of conservative talk radio and cable outlets to combat perceived liberal bias. Liberals followed suit with programming (albeit less effective) of their own.

As this cacophony crescendoed, a third trend developed as political operatives realized they had more room to stretch the truth. In 2004, an aide to President George W. Bush dismissed a journalist for being part of a “reality-based community” of people who “believe that solutions emerge from your judicious study of discernible reality.” But even Mr. Bush believed there were limits to truth-bending. The ads that attacked the military service of Senator John Kerry came from the ostensibly independent “Swift Boat Veterans for Truth.” After the ads aired, Mr. Bush belatedly called them “bad for the system.”

A fourth factor: most news organizations (with notable exceptions) abandoned their roles as political referees. Many resorted to an atrophied style that resembled stenography more than journalism, presenting all claims as equally valid. Fact checking, once a foundation for all reporting, was now deemed the province of a specialized few.