Click here to view "Can't Explain."

Barack Obama began airing a new TV spot (subscription) today that features arguably Joe Biden's best sound bite from last night's debate, challenging Sarah Palin's statements on health care.

During Thursday night's forum, Biden criticized John McCain's plan to give families a $5,000 tax credit for health care because it would mean taxing employer health plans. "Taxing your health care benefit," he said. "I call that the ultimate 'Bridge to Nowhere.'"

The line, a thinly veiled jab at Palin's onetime support for the now-infamous Gravina Island Bridge -- a.k.a. the "Bridge to Nowhere" -- drew a laugh from the otherwise well-behaved crowd at Washington University in St. Louis last night.

"Can't Explain" is the second Obama spot this fall to make use of debate footage: The Illinois senator hit the airwaves last week with an ad highlighting the fact that McCain did not use the phrase "middle class" in their first debate. McCain produced an ad mocking Obama for agreeing with the Arizona senator a number of times in their first meeting, but that spot was released on the Web only.