Senator Rick Santorum of Pennsylvania, the third-ranking Republican in the Senate leadership, sums up his race for re-election this year with a perverse pride: "The other side of the aisle wants to beat me more than anything you can possibly imagine," he told the Greater Lehigh Valley Auto Dealers Association not long ago.

Mr. Santorum is almost certainly right. No other race in the nation has so focused the Democratic Party's energy, resources or raw hunger to return to power on Capitol Hill. No other race so captures the Republican Party's vulnerabilities this year, with some public opinion polls consistently showing Mr. Santorum trailing his Democratic opponent, State Treasurer Bob Casey Jr.