For what I’m assuming are obvious reasons.

Via WaPo:

Joe Biden comes into tomorrow’s vice presidential debate with opinion stacked against him. According to a new Pew Research Center poll, more voters view the vice president unfavorably than favorably by a 51 percent to 39 percent margin.

Opinions on Rep. Paul Ryan (R-Wis.) are more evenly divided, with 44 percent of voters viewing the Republican vice presidential nominee favorably and 40 percent viewing him unfavorably.

Biden’s image has deteriorated significantly since his 2008 debate with then-Alaska Gov. Sarah Palin. At that time he was viewed favorably by 53 percent of voters and unfavorably by 31 percent. But Palin was also more popular at the time than Ryan is now, with 51 percent of voters viewing her favorably and 40 percent unfavorably.