A protracted nominating campaign and unreliable Senate schedules have made it difficult for the presidential candidates to make it back to Washington to vote. So difficult, in fact, that Senator John McCain of Arizona has missed more than half of the roll-call votes since January 2007, more than any other senator except Senator Tim Johnson of South Dakota, who spent most of 2007 recuperating from a brain hemorrhage. When Mr. McCain ran for president in 1999-2000, he missed just 30 percent of the Senate votes.

Still, Mr. McCain has a long way to go to match the number of votes Senator John Kerry of Massachusetts missed when he ran for president in 2003-4: 72 percent. He led the entire Senate, followed by Senator John Edwards of North Carolina, who missed nearly half the votes.