Not that he didn't have opportunities to do so. Sprinkled throughout her uneven performance Thursday, moderator Gwen Ifill asked the candidates about their visions of the role of vice president. Early on, she asked how each would "work to shrink this gap of polarization" in Washington. Later, she reminded them both about their earlier dismissive comments about the office, including Palin's now classic "what is it exactly that the VP does every day?" And Ifill even teed up the grim scenario voters must consider:

"Probably the biggest cliche about the vice-presidency is that it's a heartbeat away, everybody's waiting to see what would happen if the worst happened...How would a Biden administration be different from an Obama administration if that were to happen?"

In his reassuring response, Biden offered the prospect of strength - and continuity. But after somberly opening with "God forbid that would ever happen, it would be a national tragedy of historic proportions if it were to happen," Biden quickly moved on to describe how his agenda would be Barack Obama's.

But at that moment, Biden could have crystallized for voters the growing concerns over Sarah Palin. With a little subtlety, Joe Biden might have reminded Americans about their persistent worries about the 72 year old John McCain's age and his neophyte running mate. Boiled down to a two-word talking point, Biden might have said:

The first duty of the vice president is to be prepared to assume the presidency at any moment beginning on Day One. At a time of unthinkable tragedy and national crisis, Americans must have confidence that the new occupant of the Oval Office can protect and lead the nation. Whether President Biden or President Palin, that is the greatest responsibility and gravest challenge we each must be ready to face.

To be sure, the "readiness gap" Americans perceive between Biden and Palin is real and dramatic. Heading into Thursday's debate, a Pew poll found that only 37% of respondents felt Palin was qualified to step in as president. A CBS poll of undecided voters immediately after last night's contest showed that 91% said Senator Biden could be effective as president versus only 44% who believed Governor Palin could serve as president if needed.

For her part, Sarah Palin last night accomplished what she needed to. After her serial disasters with Katie Couric and Charles Gibson, a Palin meltdown last night could have effectively ended the election. But her performance - and it was merely a performance - provided a lifeline, if perhaps only temporary, for a battered John McCain.

As for Joe Biden, he had a strong outing and was viewed as the clear winner. But his commanding presentation was missing the two words that might have changed the trajectory of the campaign.

President Palin.