John McCain‘s bombshell vice presidential pick could be a minefield for Barack Obama‘s No. 2, Joe Biden.

Top Biden selling points were his quick wit, sharp tongue and ready ability to pounce like a political pit bull on John McCain and his then-unknown veep partner.

Some observers think the best line of this election was when Biden, in an early presidential debate, zinged Rudy Giuliani, quipping, “There’s only three things he mentions in a sentence: a noun, and a verb and 9/11.”

Team Obama salivated over the Delaware brawler similarly skewering the likes of, say, ex-Massachusetts Gov. Mitt Romney in the vice presidential debate this fall, or perhaps Sen. Joe Lieberman, the ex-Democrat from Connecticut.

“Biden is an attack dog, but he’ll need to keep his fire aimed at McCain,” said Zelizer. “If he is too aggressive or makes one of his infamous quips, he will bring back many of the bad feelings [among Clinton backers] that the Democratic convention sought to erase.”