He’s the only member who voted against last week’s debt deal, and also the only one who wrote his own individual budget this year — both of which make Sen. Patrick J. Toomey the wild-card selection to the 12-member deficit supercommittee charged with finding $1.5 trillion in deficit cuts by Thanksgiving.

Mr. Toomey is not a chairman or ranking member of any committee, he is not an elected caucus leader, nor does he bring the experience as a former White House budget director that his fellow freshman Republican committeeman, Sen. Rob Portman, carries.

What the first-term Pennsylvanian does have, though, is a long record of fighting for spending cuts — and even leading the charge against his own party during his six years in the House, from 1999 through 2004, when he stepped down to fulfill a term-limits pledge he made to his district...