“Some thought they couldn’t do it this year, but with their fans standing strong behind them, they defied the odds on several occasions.”
 — Rick Santorum on the Pittsburgh Steelers.

Sen. Santorum has been watching football and has learned a lesson. If an underestimated team from western Pennsylvania can win the Super Bowl, then maybe an underestimated Republican senator from western Pennsylvania can win reelection. But for Santorum to win, he would not just have to “defy the odds” but rewrite the laws of physics, or at least of political science.

Rick Santorum is the most vulnerable Senate incumbent by far. But there are still those who defend his chances in the fall, noting his close races and come from behind ability.

Those claims, however, bump up against a stark reality. Santorum does not just have one problem that he must overcome. All of the stars are aligned against him.

In the past, Santorum has not faced the strongest opponents, but he will in November. And his polling numbers are the worst he has ever had and worse than other Senate incumbents who have lost.

Social scientists have begun to examine the role of the quality of the opponents as a factor in determining election prospects. To put it simply, senators who come into office by knocking off strong, long-term incumbents are likely to be strong candidates themselves in the future. Conversely, senators who win in open seats or beat weak, one-term incumbents, may not be strong candidates themselves and may be vulnerable down the road.

Santorum won his first Senate election by beating Democrat Harris Wofford, an appointed senator who had then won a special election in 1991. Santorum beat Wofford 49-47 percent in the Republican tidal wave of 1994. In 2000, Santorum faced Ron Klink, a congressman who had never held statewide office, beating him 52-46 percent.

This time, Santorum is facing a tough opponent. Bob Casey Jr.’s father was a very popular governor. The candidate has held statewide office himself. He is a moderate, closer to Santorum on abortion and guns than he is to other Democrats. He even supported the confirmation of Justices John Roberts and Samuel Alito. And Democrats are solidly united behind him.

Santorum’s poll numbers are abysmal. Recent polls show his job approval rating at 40 percent or below and his disapproval rating nearly 50 percent. Typically, an approval rate that dips below 50 percent is a sign of big trouble. Santorum also trails Casey by 10-16 points in head-to-head match-ups.

Santorum has never been in such bad shape before. Polls in the fall of 1994 showed him leading Wofford. In 2000, he led Klink by good margins in every poll and Election Day was closer than anticipated. His disapproval rating was below 30 percent.

And if you look at recent incumbent senators who have lost in the general election in recent years, Santorum is much worse off. In 2004, Senate Minority Leader Tom Daschle (D-S.D.) lost to Republican John Thune but never trailed by much in the polls. In 2002, Sen. Max Cleland (D-Ga.) led in the polls before losing to Republican Saxby Chambliss.

Perhaps the best parallel is former Sen. Tim Hutchinson (R-Ark.) in 2002, who faced Mark Pryor, a statewide office holder and son of a political icon. But Santorum’s polls are even worse than Hutchinson’s. Pryor beat Hutchinson 54-46.

If you bet on the Steelers last weekend, you made some money. If you believe in parallels between football and politics, your bookie would be happy to take your money.

Fortier is a research fellow at the American Enterprise Institute.