No one will be more relieved than John McCain if a major crisis erupts on Thursday to overshadow the debate between the vice-presidential candidates, Sarah Palin and Joe Biden.

Alaska's one-term governor is hugely popular with voters, but she is also a liability for the Republican ticket, as her painful-to-watch interview last week with Katie Couric of CBS News showed. Her woeful inexperience and the yawning gaps in her knowledge of basic issues reminded voters of McCain's advanced age at 72, and sparked questions about what sort of president she would make. Her performance prompted one right-wing commentator, Kathleen Parker of the 'National Review', to call on her to quit. "Only Palin can save McCain, her party, and the country she loves," she wrote. "She can bow out for personal reasons, perhaps because she wants to spend more time with her newborn."

That explains why the McCain campaign did not want his running-mate anywhere near Friday's presidential clash in Oxford, Mississippi. Instead, Palin watched it on TV in Philadelphia, while Senator Biden gave countless interviews in the media "spin room" outside the Obama-McCain debate.

However, the CBS interview was buried under a torrent of news about the Wall Street bailout plan. In fact, running-mate debates rarely matter: in 1988 the Democratic candidate, Lloyd Bentsen, smacked down Dan Quayle, telling him, "You're no Jack Kennedy", after the Republican tried to don the mantle of the assassinated President. But the evening was declared a success for Quayle, and the Republicans won the election.

Woody Allen's remark – "Eighty per cent of success is just showing up" – may be enough for Palin. Indeed, Biden must be careful to avoid humiliating her too badly, for fear of being called a sexist bully.