But I said there were other possible scenarios. I can think of three.
The 1964/1972 scenario: Challenger disqualifies himself. Barry Goldwater and George McGovern were idealistic, intelligent senators who took positions on issues that made them unacceptable to most voters in years favorable to incumbents.
This could happen to Mitt Romney this year. And it might well have happened if some of his primary opponents had won the nomination. But he doesn't seem to be the kind of candidate who would disqualify himself. Chances for this scenario: less than 5 percent.
The 1988 scenario: Affluent voters break strongly Republican. Vice President George Bush was 17 points behind Michael Dukakis after the Democratic National Convention. But he came back to win by a 53 to 46 percent margin.
One reason is that his "read my lips, no new taxes" promise solidified his support among affluent suburbanites. His margins in suburbs enabled him to carry metro Philadelphia, metro Baltimore, metro Detroit, metro Chicago, metro Los Angeles and the surrounding states.
Since then, affluent non-Southern suburbanites have trended Democratic. And big city crime and welfare rolls -- cause for complaint in 1988 -- have declined.
Republicans' conservative stands on cultural issues and the increasing Southern influence in the party repelled suburbanites. Barack Obama carried most affluent non-Southern suburbs handily in 2008.
But Romney showed particular appeal to this constituency in the primaries. Without big margins in affluent suburbs, he would have lost Michigan, Ohio and Illinois to Rick Santorum.
Romney's proposed tax cuts and Obama's proposed tax increases pose the sharpest contrast on the tax issue since Bush beat Dukakis 24 years ago. And economics is far more important than cultural issues this year. Chances for the 1988 scenario: maybe 20 percent.
The 1980 scenario: Late break away from the incumbent. We remember the 1980 election as Ronald Reagan's landslide defeat of Jimmy Carter.
It didn't look like that during the campaign. Carter led in polls much of the time. Sometimes the race looked like a 2000/2004-style long, hard slog through target states.
But Carter's job rating was buoyed up that year by approval of his varied attempts to free the hostages in Iran. Underneath those numbers, his ratings on other foreign issues and the economy were weak.