WASHINGTON--Like many members of the House of Representatives, Pat Toomey, a 41-year-old Pennsylvania Republican, hankers to move to the chamber on the other side of the Capitol. But he has a wee problem. Pennsylvania already has two Republican senators. Rick Santorum, 45, is a rising star--he is chairman of the Senate Republican Conference, the third-highest leadership post--midway through his second term. The other Republican senator is Arlen Specter, 73, who next year will seek his fifth term. Toomey, who term-limited himself by pledging to serve only three terms and who is in his third, says, with sincere serenity, he will...