DERRY, N.H. — Rick Santorum’s ugly 2006 Senate reelection loss is going to keep casting doubt on his electability in the 2012 presidential race.And a look at the numbers from that Pennsylvania race shows why.

It was pretty bad.

Republican presidential candidate Rick Santorum at a campaign event Sunday in Greenville, S.C. (AP Photo/David Goldman)

While Santorum won the 18-to-29-year-old demographic in both his 1994 and 2000 Senate campaigns, he took just 32 percent of them in 2006. The same happened with white Catholics, who supported Santorum twice before giving him just 42 percent of the vote in 2006.

But here’s the key number: While he won independents 57 percent to 43 percent in his 2000 reelection campaign, he lost them 71 percent to 28 percent in 2006 — a pretty stunning 29-point dropoff.

Now, it should be noted here that 2006 was a very bad year for Republicans, which is at least partly why Santorum’s numbers dropped so precipitously. Santorum was also running against Casey, a man whose name is worth a lot in the Keystone State.

In addition, Santorum was left for dead rather early by the national Republican Party, which stopped running ads on his behalf a few weeks before the election because he appeared to be a lost cause (another state where the party pulled out early, Ohio, featured a similar blowout of a GOP incumbent).

Santorum, who is often asked about his electability, also points out that, before 2006, he won races despite some pretty tough electorates.

“I’ve run five times, and every time (I’ve had) the odds against me,” he said at an appearance at a diner here this afternoon. “And I’ve won four out of my five races. Who’s the most electable? It’s an absurd statement.”

But the fact is that Santorum never polled well in that 2006 race and never really had much of a chance. He trailed by double digits as early as mid-2005, when few imagined the Democratic wave that was approaching.

While he came into Congress in a pretty heavily Democratic district and won two Senate races in a nominally blue state — fair points that Santorum raises in his defense — toward the end of his Senate tenure and today, he has become a pretty divisive character.

A look at one last crosstab shows why.

In his 2000 race, Santorum took 19 percent of the vote from registered Democrats — a pretty good clip for any Republican and undoubtedly capitalizing on a network of more conservative Democrats in the western part of the state, where Santorum is from.

In his 2006 race, he took just 7 percent of Democrats.

Essentially, all of Santorum’s support in his 2006 reelection bid came from registered Republicans, along with a few independents who probably vote very reliably for the GOP and might as well be registered with the party.

The crossover appeal that Santorum showed in his previous races was gone, and for a Republican Party looking for someone who can win over independents, he might not be their guy.

Updated at 2:16 p.m. with Santorum’s remarks from Derry.