Presidential candidates usually capture a percentage of the vote that is about the same as their approval rating. In 2004, George W. Bush’s share of the vote was about equal to his job approval number right before the election; Mr. Clinton, in 1996, and George H.W. Bush in 1992, were about within that margin of error. That suggests Mr. Obama would be in trouble in a two-person race.

Yet there is another indicator that gets less notice: the status of the challenging party. The last four times an insurgent party has captured the presidency — 1980, 1992, 2000 and 2008 — it had a positive approval rating a year before Election Day that usually was better than the incumbent party’s…

Today, a plethora of polls show that both parties are held in low regard by the electorate, but the Republicans consistently do worse. They have been on a steady decline throughout this year. The explanation Republican politicians give for this slide is instructive. National politicos say congressional Republicans are to blame; on Capitol Hill, there is a lot of finger-pointing at the presidential candidates.