When Joe Biden was last seen on a national debate stage four years ago, it was put to him by the moderator Gwen Ifill that his perceived weakness was lack of discipline. "You're very kind suggesting my only achilles heel is my lack of discipline," the then-senator replied. "Others talk about my excessive passion."

On Thursday night, as he steps in front of the TV cameras in Danville, Kentucky, to face his vice-presidential rival Paul Ryan, he will need to overcome both impediments and display discipline combined with great, though not excessive, passion. The stakes are high: in the wake of Barack Obama's own debate appearance last week the president's poll ratings have taken a nose-dive and the Romney-Ryan ticket has been reinvigorated.

It falls now to Biden, a politician who, as vice-president, has spent much of the past four years out of the limelight, to deliver the performance of his political career. "He has to reframe the debate," said Darrell West, a specialist in government and media at the Brookings Institution.

"Obama's poor showing allowed Mitt Romney to completely recast his record, and the Democrats can't afford that to happen again. I expect Biden to be aggressive in pointing out the differences between the two campaigns over the major issues."

For the past three days, Biden, 69, has been sequestered at home in Wilmington, Delaware, in intensive debate prep with a team of advisers led by David Axelrod, Obama's chief re-election strategist. Biden has been rehearsing alongside Maryland congressman Chris Van Hollen, who has been role-playing Ryan, and has been thoroughly mugging up on his opponent's policy positions including a close textual analysis of the book co-written by Ryan: Young Guns.

The implication is clear: Biden will move heaven and earth not to repeat Obama's mistakes of last week in which the president was deemed to have been too passive in his encounter with Romney. "He's going to be very attuned to any attempt by Ryan to present himself as more moderate than he is," West said.

The need for Biden to exploit the contradictions in his opponent's positions – in a way that Obama failed to do with Romney – is clearly on the minds of top aides. Jennifer Psaki, press secretary of the Obama re-election campaign, told a gaggle of reporters on Air Force One this week that "the question here is: which Paul Ryan is going to come to the debate later this week? Is it going to be the Paul Ryan who has been misleading about everything from his marathon time to details and specifics he included in his convention speech?

"Or is it going to be the Paul Ryan who has eagerly embraced voucherizing Medicare and tax cuts for millionaires and billionaires? We'll all be watching."

Robert Barnett, the doyen of debate prep coaches, who has held the hands of Democratic candidates in no fewer than eight presidential races since 1976 and is part of the Obama-Biden prep team this year, said the focus on substantial policy differences is always ultimately more important than the one-line zinger or the gaffe. Barnett points out that the most famous soundbite of any VP debate – "Senator, you're no Jack Kennedy" – was delivered against Dan Quayle by Lloyd Bentsen who, despite that exocet remark, went on to lose with Michael Dukakis the 1988 election.

Barnett also points out that in last week's debate between Romney and Obama "there was a clear contrast in major issues and not a lot of focus on one-liners. I think that makes for a better debate, when the discussion is substantive."

With that in mind, Biden is unlikely to repeat Obama's inexplicable failure to raise the Romney's record at Bain Capital, the asset management firm he set up, and the Republican nominee's notorious comments about the dependent 47% of Americans. And there will be plenty to go at in terms of the record of his immediate opponent.

As an avowed policy wonk who refers to himself as a "PowerPoint guy", Ryan, 42, brings to the debate a wealth of policy formulations that give him impressive Beltway credentials but leave him exposed to Democratic attack.

Top of the pile is the Paul Ryan budget, which runs counter to Romney's claim last week that he intends to leave unchanged taxes for the most wealthy Americans. Ryan's budget, by contrast, calls for a tax cut for the super rich to 25% and an effective increase in taxes for the most poor by removing tax breaks for low-income earners.

Then there's Medicare, the national healthcare insurance scheme for over-65s, which Ryan has proposed essentially to privatise for anyone under 55. He's been similarly bold in his thinking about Medicaid, the national health insurance scheme for poor families, drawing up plans that independent analysts have suggested would lead to the slashing of the Medicaid budget by a third over the next decade.

West expects Biden, in addition, to raise Ryan's controversial views on abortion – another area in which the running mate sits uneasily with the new, moderate Romney that was unveiled in Denver last week. All Biden has to do is point out that Ryan backed legislation that would grant a fertilised egg the full legal rights of a person, and that his co-sponsor of the bill was none other than Todd Akin, the Missouri Republican who in August astonished the world with his statement that women who had been "legitimately" raped rarely got pregnant.

"The abortion issue is a good way for Biden to reach out to women voters who are wavering in their support for Obama," West said.

One thing that Biden cannot assume, however, is that Ryan will be taken off guard by questions surrounding his positions. Though inexperienced in national debating, the Wisconsin congressman has been engaged for days in his own deep debate prep and has said he is braced for Biden going on the offensive. "I expect the vice-president to come at me like a cannonball," he told the conservative Weekly Standard.

It all makes for a potentially barnstorming debate after the rather drab presidential affair last week. Joe Biden has achilles heels aplenty, but being dull is not one of them.

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