The Sunday Telegraph By

WASHINGTON: As she takes the stage at the Wynn Hotel in Las Vegas this week, Hillary Clinton will be possibly the best-prepared US presidential debate candidate in history.

For months, she has been holding secretive dress rehearsals for what will be her first televised debate in the Democrat presidential candidacy contest. In each "prep" session, her lawyer, Bob Barnett, has played the role of her opponent, Bernie Sanders, the maverick Left-wing senator from Vermont, hitting her with questions on every possible topic.

But despite the carefully plotted lines of attack, and the seemingly spontaneous one-liners crafted to undermine Mr Sanders, the Clinton campaign machine is about to encounter the one thing it can't plan for - the element of chance.

For unlike most other American politicians, Mr Sanders, a self-declared democratic socialist who is seen as America's Jeremy Corbyn, does not script his lines in advance. Indeed, he may only know what he is going to say when it comes out of his mouth.

Which means that no matter what Mrs Clinton does, Tuesday's encounter will have an element of Las Vegas style roulette to it - with Mr Sanders, a rank outsider, enjoying slightly more favourable odds than he might otherwise have done.

What is guaranteed is a show as entertaining as anything at Caesar's Palace, as a man who prides himself on being passionate and freewheeling comes up against a woman who is seen as anything but.

Viewers may well be hoping for a repeat of Mr Sanders's performance in a televised debate in 2006, when he leapt from his chair to jab a finger in the face of Richard Tarrant, a wealthy Republican businessman, and shouted "people like you ..."

Mr Tarrant eventually lost to Mr Sanders in that year's Vermont Senate election, with the loser admitting that his rival's "fieriness was part of the attraction".

The fact is, nobody knows what will happen on Tuesday - least of all Mr Sanders's own camp. He has had no one playing Mrs Clinton in practice sessions, because there are no practice sessions. According to aides, he will "just be himself".

"Maybe this is a little unorthodox, but I do believe there's such a thing as being overprepared," said Tad Devine, Mr Sanders's chief strategist. "Bernie's just going to go 'come on'. We don't want to stiffen him up. He is who he is."

By all accounts, Mr Sanders didn't start thinking about how to handle the debate until 10 days ago, when his advisers asked him if he had a plan. He didn't. The candidate then asked for a few briefing papers, and to talk to a couple of experts on foreign and military issues.

Meanwhile, rather than resting up for the big night, he has refused to break stride from his campaigning before the debate, holding events in Colorado and Arizona.

Such "Feel the Bern" rallies have been attended by tens of thousands of people at a time and he is surging in polls, particularly with young Democrat voters.

Mrs Clinton still leads by 41 per cent to 25 per cent, while the vice-president, Joe Biden, who has yet to decide if he will enter the race, is on 19 per cent.

But rather like a highly-paid QC taking on a litigant-in-person in court, Mrs Clinton will have to be careful that her efforts to run rings around him do not end up making him seem like a bullied underdog.

In terms of policy, Mrs Clinton has already been laying the groundwork to see off the challenge from Mr Sanders, an ex-hippy who is one of the few genuine Left-wingers on the mainstream political scene in the US.

She has declared opposition to Barack Obama's Trans-Pacific Partnership trade deal, which is opposed by unions claiming it will cost jobs. She has released a detailed plan to crack down on Wall Street, responding to criticisms that she has been too close to bankers.

None of which fazes Mr Sanders's campaign manager, Mr Devine. He insists that while Mr Sanders has never been in front of such a huge audience before, he will not be overawed. "It can have a big impact," he said. "But different candidates prepare in different ways."

Nor, once the debate is all over, will Mr Sanders be analysing how he has done too closely. Unlike Mrs Clinton, he doesn't employ a pollster.