Q What is at stake?
A For Biden, the challenge will be to get the Democratic ticket back on solid footing after President Obama's less-than-stellar performance at the presidential debate last week in Denver. For Ryan, the objective will be to keep up the enthusiasm and energy that seems to be following Mitt Romney after he performed much better than expected in Denver.
Q What must Biden accomplish?
A Biden's mission will be to get the Democrats back on message, so look for him to go on the attack. He will likely make the case that Romney's tax plan benefits the wealthy and hurts the middle class. He will probably defend the new health care law. And he will likely go where Obama did not go in Denver and attack Romney over his claims about 47 percent of Americans.
"I think you'll see the vice president take the gloves off, and I think it's going to be pretty hard hitting," said Sean Sutton, an expert on presidential races and chairman of the political science department at the Rochester Institute of Technology in New York. "He's really going to go after Mr. Ryan, and he's going to try to make the case that they've been making all summer -- that Mr. Ryan and Governor Romney are not up to the task."
Q What must Ryan accomplish?
A Though Ryan has served in Congress for 14 years, he is still largely unknown to most voters outside his home state of Wisconsin. Ryan must introduce himself to a national audience and show that he is ready to step into the presidency in the event that something should happen to Romney.
"The most important job for him is to make an emotional connection to voters -- and by emotional connection, I mean present himself as a likeable and trustworthy person," said Russell Muirhead, associate professor of democracy and politics at Dartmouth College.
Q What must Biden avoid?
A Biden can be long-winded and speak off the cuff, often trampling Obama's message in the process.
"Biden is at his best when he's crisp and disciplined," Muirhead said. "He's such a generous speaker that he sometimes relaxes that discipline and tries people's patience. He needs to avoid speaking in an undisciplined way."
In other words, "he has to avoid becoming a joke," said Wayne Fields, an English professor at Washington University in St. Louis and the author of a book on presidential rhetoric.
Q What must Ryan avoid?
A During his tenure in the House, Ryan often has been at the center of some of the most bitter, partisan debates over fiscal policy. In debating Biden, Ryan must moderate those positions for a national audience without losing the conservative backing that helped him land a spot on the presidential ticket in the first place, Fields said.
"He has to demonstrate maturity -- that he's ready to learn, not just ready to pronounce," Fields said. "He has to avoid looking reckless."
MICHAEL COLLINS OF SCRIPPS HOWARD NEWS SERVICE