WASHINGTON - VICE-presidential debates rarely play a role in White House races, but the showdown between Republican Gov Sarah Palin and Democratic Sen Joe Biden will be different - it could matter.

The highly anticipated encounter on Thursday between the verbose Mr Biden and the sheltered Mrs Palin will likely draw a larger television audience than last week's first debate between their bosses, presidential rivals Barack Obama and John McCain.

The spotlight will be on Mrs Palin, the unknown moose-hunting mom from Alaska whose surprise choice shook up the White House race, made her a political celebrity and raised concerns about her readiness to step into the top job - the prime function of the vice-president.

Since then, Mrs Palin has been protected from reporters, giving just three interviews and holding no news conferences. It is a strategy that has only intensified the concerns, raised the stakes for her first unscripted performance and made her the butt of late-night comedy jokes.

'Vice-presidents normally don't matter, but there is a threshold they must cross to prove they can step in and be president,' said Mr Mitchell McKinney, a communications expert at the University of Missouri.

'The bar is low, but if she doesn't cross that threshold it could damage Mr McCain,' he said.

On paper, the pairing looks like a mismatch. Mr Biden, 65, the talkative but unpredictable chairman of the Senate Foreign Relations Committee, is one of the most knowledgeable Democratic foreign policy experts.

He will try to reassure voters who still harbour concerns about the relative lack of experience for Mr Obama, a first-term senator from Illinois who has a slight lead in opinion polls ahead of the Nov 4 election.

Mrs Palin, 44, a self-described hockey mom, has seemed hesitant and heavily coached in her interviews. She must reassure voters who are worried about her being a heartbeat away from the presidency under the 72-year-old McCain, an Arizona senator.

''Low expectations'
'It will be pretty easy for her to exceed expectations - they couldn't be any lower,' said Mr David Steinberg, a debate coach at the University of Miami in Florida who recently watched tapes of her 2006 debates in the race for governor.

'She was very articulate, certainly competent and assertive,' he said. 'She was quite good.'

The McCain campaign has been quick to attack anyone who criticizes Mrs Palin, accusing them of sexism or condescension.

Mr Biden will have to walk a fine line in not appearing too assertive with her.

'I don't want to be Joe Biden on debate night. He can't be a bulldog and he can't be a wallflower. He can't be too knowledgeable and he can't be too passive,' said pollster John Zogby. 'He'll be like the dad up there with his daughter - and dads never win.'

Mr Biden was a forceful debater throughout the long Democratic primary season, when he unsuccessfully battled Mr Obama for the nomination. But Mr Steinberg said Mr Biden was unlikely to challenge Mrs Palin directly and would take his case straight to voters.

'He should look at the camera, kind of like Mr McCain did on Friday night, and avoid a direct interchange with her. He'll try to focus the debate on Mr McCain, not her,' he said.

But Mr Biden has been mistake prone, most recently when he referred to a televised speech by President Franklin Roosevelt after the stock market crash of 1929 - four years before Roosevelt took office and long before television became widely available.

'The fear for Democrats is that Joe Biden will slip up and say something he shouldn't and patronise her. That is exactly what they are working to avoid in practice sessions,' Mr Steinberg said.

Vice-presidential debates rarely produce campaign-changing moments, although they have featured memorable performances. In 1988, Democratic Sen Lloyd Bentsen delivered his classic one-liner to Republican Sen Dan Quayle: 'You're no Jack Kennedy.'

The 1992 debate featured the befuddled performance of independent candidate Ross Perot's running mate, Adm James Stockdale, who wondered aloud: 'Who am I? Why am I here?' Both candidates are taking breaks from the campaign trail to practice for their encounter.

Mrs Palin plans two days of debate preparation at Mr McCain's Arizona retreat.

The campaigns have tried to shape expectations, and Mr Obama aides said they had seen tapes of Mrs Palin's debates during her run for governor.

'She's a terrific debater. She has performed very well. We expect she'll give a great performance,' Obama campaign manager David Plouffe said. -- REUTERS