I'll post the video if I can find it... [update: video now embedded]

A member of the audience at one of Mr. Santorum’s campaign events recently stated,

“[President Obama] is an avowed Muslim... he has no legal right to be calling himself President.”

Rather than addressing the claim, Mr. Santorum promised only that he was doing his best to get Obama out of the White House.

Here’s a more appropriate reply:

“First, President Obama is not a Muslim. He’s a Christian.
“But, even if he WERE a Muslim, our Constitution explicitly states that, ‘no religious test shall ever be required as a qualification to any office or public trust under the United States.’
... even if he WERE a Muslim, he would still have every right as a U.S. citizen to be President.
“Finally, I will do my very best to make sure that, with your help, we get him out of the White House.”

... but, no: Mr. Santorum let the claim stand, without challenge, without correction.

During a similar campaign moment in 2008, Senator McCain had the courage to correct the speaker: No, Obama is not a Muslim.
(... and it was General Colin Powell who provided the follow-up: but even if he were, so what?)

Whatever good will I might have felt for Mr. Santorum was lost in an instant.
Mr. Santorum does not deserve to be President if he can’t stand up for and defend the truth, and defend the Constitution.