AP

UNDER normal circumstances a vice-presidential debate is a snooze. Voters care about the top of the ticket. Unless the running-mate goes berserk, all will be well. But the conventional wisdom goes out the window when one of the candidates is a culture warrior in collapse. The past few weeks have been hard on Sarah Palin, the running-mate of John McCain. In one disastrous interview the Republican vice-presidential candidate was unable to name a Supreme Court decision other than Roe v Wade. So interest in the debate on Thursday October 2nd between Mrs Palin and Joe Biden, the running-mate for Barack Obama, was unusually high. Republicans girded themselves for grim news. Democrats were cautiously optimistic. Some sensed a trap. Mr Obama's campaign manager insisted that Mrs Palin was “one of the best debaters in American politics.”

The good news for the McCain campaign is that Mrs Palin was confident and assertive, and made no big errors. The bad news is that a mediocre performance counts as good news. But they will take what they can get at this point.

The McCain campaign had lobbied for a constricted debate format: two-minute questions, without long freewheeling exchanges. This worked well for Mrs Palin. It seemed that she had memorised some talking points and was determined not to deviate from them. “And I may not answer the questions the way that either the moderator or you want to hear,” she said, near the beginning. “But I'm going to talk straight to the American people and let them know my track record also.”

This was effective, if blunt. One telling exchange came when the moderator, Gwen Ifill, asked Mr Biden whether Americans have the stomach to put troops on the ground in Darfur, a war-torn region in western Sudan. Mr Biden noted his early advocacy for an intervention in Bosnia, the objections that were raised and the eventual success of the mission. He concluded that America should rally the world to intervene in Darfur.

Mrs Palin offered a response first to comments on Iraq. “Oh, yeah, it's so obvious I'm a Washington outsider,” said Mrs Palin. “And someone just not used to the way you guys operate. Because here you voted for the war and now you oppose the war.” She went on, however, to agree that a no-fly zone should be supported in Darfur.

At other points Mrs Palin was baffling. Mr Biden criticised the Republicans for having too much faith in drilling as a solution to the energy crisis. “John McCain has voted 20 times against funding alternative energy sources and thinks, I guess, the only answer is drill, drill, drill,” he said. “The chant is 'drill, baby, drill',” she corrected, playing some kind of gender card against herself.

Both of the vice-presidential candidates, like their running-mates, were sour on Wall Street and big on Main Street. They tussled as they tried to prove their credentials. Mr Biden said that he has a friend who cannot afford to fill his car with petrol. Mrs Palin talked about sidling up to other parents at the football field and offered “a shout-out to all those third graders at Gladys Wood Elementary School.” Mr Biden movingly defended his case: “Look, I understand what it's like to be a single parent,” he said. “When my wife and daughter died and my two sons were gravely injured, I understand what it's like as a parent to wonder what it's like if your kid's going to make it.”

Mr Biden may also have benefited from low expectations. Democrats had feared that he would seem condescending or pompous. Republicans had chortled over the same possibilities. No one gave him much credit.

But Mr Biden gave a strong and disciplined performance. He was self-effacing and downplayed his past disagreements with his running-mate. He shrugged off their mismatched votes on the bankruptcy bill by saying that “Barack Obama saw the glass as half-empty. I saw it as half-full”. He was affable but aloof to Mrs Palin, and kept his sights trained on the main target. He said that he loves his friend John, but that Mr McCain has the same policies as George Bush and is not a real maverick. The voters who were mesmerised by Mrs Palin may have been bored to hear so much about Mr McCain. But as Mr Biden knows, boring is not such a bad trait in a running-mate. The Republicans have learned that one the hard way.