A New York Times Week in Review piece yesterday seems to attribute various election outcomes to widely publicized negative attacks:
For raw, crushing smear power, the 1964 “Daisy” ad, made for President Lyndon B. Johnson’s campaign and suggesting that the election of the Republican candidate, Barry Goldwater, would mean the end of life on earth, has still never quite been equaled.
And the 11th-hour telephone “survey” of Republican primary voters in South Carolina in 2000, asking “Would you be more or less likely to vote for John McCain if you knew he had fathered an illegitimate black child?” will probably keep its place on the Mount Rushmore of smear for a while.
...The unending news cycle, the explosion of the blogosphere and the freelance work of independent groups like the Swift Boat veterans of 2004, whose campaign severely undercut John Kerry’s bid for president, has made every campaign entourage a kind of road crew cum paramedic team.
... The rhythm of campaigning may have quickened, but the notion that a counterpunch delayed was a counterpunch denied did not seem to take hold in conventional wisdom until 1988.
That year, Gov. Michael Dukakis of Massachusetts fell under the wheels of a negative campaign juggernaut — watching from a dignified remove as the supporters of George H. W. Bush wiped out Mr. Dukakis’s 17-point lead by defining him as the man who furloughed the rapist-killer Willie Horton.
While I obviously have normative concerns about misleading campaign attacks, it's much less clear that the LBJ ad had "crushing smear power," that the Swift Boat ads "severely undercut" John Kerry, or that Michael Dukakis lost his 17-point lead in the polls as a result of the Willie Horton ad. The leading models of presidential elections predicted that Goldwater, Kerry, and Dukakis would lose. Journalists tend to construct post hoc narratives based on dramatic visuals from debates and campaign ads, ignoring the fundamentals that actually drive elections (the state of the economy, presidential approval, war casualties, etc.).
Update 8/19 3:49 PM: Last sentence edited for clarity and style.