Similarly, Biden has a reputation for foreign-policy expertise, and Ryan has little experience in this area, so the vice president may be tempted to try to show him up. It could backfire, though, since Biden's reputation is undeserved. Biden's record on Iraq - opposing the first war and supporting the second one; opposing the surge and proposing to split the country in three - doesn't suggest great judgment. And Ryan is fluent on foreign policy even if it hasn't been his focus.
The reaction to Obama's debate loss may also point Biden in the wrong direction. Among liberals, the prevailing view is that Obama lost because he didn't call Romney on his outrageous lies. Democrats will urge Biden to be more combative.
This probably misunderstands why Obama lost. The real problem was that he was less up to speed on the arguments and counterarguments. If Biden internalizes the Democratic conventional wisdom, he will be more engaged than Obama was - but it won't help unless he is also better informed. An amped-up yet inadequate response can come across as bluster.
A few things may work in Biden's favor. Jonathan Haidt, a professor of ethical leadership at New York University's Stern School of Business, points out that liberals are more apt to inaccurately stereotype conservatives' values than vice versa. Perhaps this lack of familiarity with the conservative worldview hurt Obama: He didn't know what to say when a Republican made a reasonable argument. Maybe Biden - who has built closer relationships with conservatives in the Senate - will have more effective rebuttals.
Democrats probably don't need to worry about Biden's penchant for gaffes. Yes, he said the middle class "has been buried the last four years," which is maybe not the ideal message for a ticket asking for four more. And he has a history of racially insensitive remarks - about Indian Americans owning 7-Elevens, for example, or Obama being "clean" and "articulate." As Ryan has pointed out, though, Biden typically makes such blunders when he is relaxed, not in high-pressure situations. He didn't say anything disastrous in his debate with Sarah Palin in 2008, which he is generally thought to have won.
Biden gave a stronger speech at the Democratic convention than Obama did. He will have to do better than Obama again this week. The race is a dead heat, according to the most recent polls, and the Democrats have been running against Ryan as much as they have against Romney. So the stakes are higher for Biden than they usually are in vice presidential debates. It's safe to say Republicans are looking forward to this more than Democrats are.
Ramesh Ponnuru is a Bloomberg View columnist and a senior editor at National Review.