John McCain last night wasted no time in trying to attack Barack Obama's weakest spot - the seething resentment some Democrats have over his treatment of Hillary Clinton.

John McCain last night wasted no time in trying to attack Barack Obama's weakest spot - the seething resentment some Democrats have over his treatment of Hillary Clinton.

As the political focus of America, and much of the world, this week falls on Denver for the Democratic National Convention, the 72-year-old Republican candidate fired off a television advert, suggesting that Mr Obama had cruelly snubbed his one-time rival in favour of Joe Biden, the senator from Delaware, as his running mate.

Knowing the wound that the bitter Democratic race for the party's presidential nomination inflicted and knowing that one in four supporters of Mrs Clinton are minded to vote for Mr McCain, the Republican candidate's aim in his TV advert was clear - stir up as much resentment as possible to destabilise a week of Obamania beside Colorado's Rocky Mountains. The McCain advert has Mrs Clinton criticising Mr Obama's tactics in the primary contest, including: "Senator Obama's campaign has become increasingly negative."

An announcer adds: "She won millions of votes but isn't on the ticket. Why? For speaking the truth."

Last night, Kathleen Strand, a spokeswoman for Mrs Clinton, insisted the New York senator's support for her colleague was clear and unambiguous. "She has said repeatedly that Barack Obama and she share a commitment to changing the direction of the country, getting us out of Iraq and expanding access to healthcare. John McCain doesn't. It's interesting how those remarks didn't make it into this ad."

Mr Biden will speak on Wednesday, when he accepts the vice-presidential nomination, during the themed day of "securing America's future". It is thought Mr Obama chose the 65-year-old Senate veteran to bolster his foreign policy credentials and to counter the Republican attack that the Democratic candidate and one-term senator from Illinois is weak on overseas affairs and lacks experience.

Plus, Mr Biden is a streetfighter, a working class Roman Catholic who has spent more than three decades in the cauldron of the US Senate. He will use this week to return Republican punches, something the nonchalant "no drama Obama" has appeared reluctant to do.

Senior Democrats quickly rallied round Mr Obama's choice of VP with Nancy Pelosi, the House Speaker, saying Mr Biden offered "the full package". She noted that during the primary contest Mr Biden criticised Mr Obama, which, she argued, showed "he's not just choosing a yes man but a person who will speak what he believes".

David Axelrod, an adviser to Mr Obama, told ABC: "He has a high regard for Senator Clinton but he felt that Senator Biden would be the best fit for him at this time."

It was revealed that Mr Obama spoke to Mrs Clinton this week, no doubt trying to ensure that any bruised feelings do not spoil the show.

"Everybody's onboard," said Robert Gibbs, an Obama strategist. "It's going to drive the media crazy when we come out of this convention united."

Mrs Clinton herself will be the key primetime speaker tomorrow night when the theme will be "renewing America's promise", focusing on the economy, education and healthcare.

Bill Clinton will not miss out either. The former president will also speak on Wednesday with the possibility his oratory will overshadow everyone's contribution, possibly even Mr Obama's.

As the tens of thousands of delegates and officials throng the Pepsi Center in Denver, 47-year-old Mr Obama will use the run-up to his acceptance speech on Thursday touring the battleground states of Wisconsin, Iowa, Missouri and Montana.

The convention opens today with a keynote address from Michelle Obama and some of her relatives, explaining her husband's background as a "very American story" - one of the criticisms and fears people have about Mr Obama is that he is not American enough.

The high point of the week will be Thursday when the focus moves from the Pepsi Center across town to the Invesco Field, home of the Denver Broncos, and capable of holding 75,000 people.

In the Mile High City, Mr Obama will accept his party's nomination on the 45th anniversary of Martin Luther King's "I have a dream speech". The Democrat candidate, hoping to be America's first black president, might feel the hand of history on his shoulder.