John J. Pitney Jr. is the Roy P. Crocker professor of government at Claremont McKenna College. He is coauthor of “American Government and Politics: Deliberation, Democracy, and Citizenship.”

...Although it’s theoretically possible that the electorate could turn against incumbents of both parties, such a result would be unprecedented in the modern era.

When large numbers of lawmakers go down, the bulk of the losses come from one party or the other. It’s even harder to picture a situation where voters would oust both Mr. Obama and the Republican majority in Congress. Other election scenarios are more plausible.

The Truman

The first is the Truman 1948 scenario. That is, a feisty president rallies support, wins an upset victory, and restores his party to power on Capitol Hill. But as political scientist Brendan Nyhan has pointed out, it’s a myth that Harry S. Truman won because of his “give ’em hell” style. He won because the economy was doing well for most of 1948, and Democrats had a big lead in party identification.

Neither of those conditions seems likely for 2012....