The move has come in the face of polls which put the Republican candidate ahead for the first time. But Obama's fight-back has split the Democrat party ahead of this week's national convention, with some insiders believing the new tactic undermines the Illinois senator's claim to represent a new type of politics.

Obama's team have produced a series of negative TV and radio ads which are airing round the clock in 18 states, including all-important swing states.

They accuse McCain of failing to protect American jobs, pandering to big business and being elitist and out of touch with the financial difficulties of ordinary Americans.

Until now, Obama has concentrated on portraying himself in a positive light and rising above the usual bitter verbal warfare of the campaign trail. His new approach represents victory for those within his team who have been advocating he hit back at McCain's criticisms.

It is also a tacit recognition that McCain's strategy has been working. The Republicans' willingness to play hardball from the start is credited with stalling the Obama campaign. A Reuters-Zogby opinion poll last week put McCain ahead for the first time – by five points.

For weeks the Democrat camp has been split, with one group of insiders warning Obama that he faces the same fate as failed 2004 presidential candidate John Kerry unless he responds in kind to the increasingly bitter attacks.

"There comes a time when you've got to go to the barricades and let it rip. Both Gore and Kerry were eviscerated on personal grounds by their opponents' campaigns," said Thomas Mann, a senior fellow with think-tank the Brookings Institute.

He added: "It is really important that Obama responds to the charge that he is elitist. Democrats don't want the criticisms to take hold and affect public opinion."

Backers of moves to put Obama's campaign on the offensive will have been pleased with his choice of Joe Biden as running mate, a veteran senator with a reputation for fighting tough on a wide range of issues.

As well as attacking McCain's record, the new ads go out of their way to show McCain standing next to the unpopular President Bush to ram home Obama's charge that his opponent promises "four more years" of the Bush presidency.

The new Democrat adverts portray the presumptive Republican candidate as a millionaire, out of touch with the concerns of ordinary Americans. It is a line of attack which Democrats have identified as striking a chord with voters, struggling with economic difficulties.

One ad shows McCain talking about the economy, pronouncing at a January debate: "I don't believe we're headed into a recession."

McCain's upbeat comments about the economy are juxtaposed with images of voters in Indiana, Ohio and Kentucky talking about their financial problems. "I sometimes struggle just to get the essentials, you know, the milk, the bread, the eggs," says Indiana resident Lauren Ahlersmeyer. The ad ends with the question: "How can John McCain fix the economy when he doesn't think it's broken?"

Another advert criticises McCain for opposing 'Buy American' legislation requiring the government to purchase US-made products.

The sharp words in the adverts mirror Obama's increasingly tough stance on the campaign trail.

But some insiders fear the new strategy could undermine Obama's claim to be the candidate of change. "Just because the other side is fighting dirty, it doesn't mean we have to stoop to their level," said one. "If we are going to win this election then we have to live up to our promise that we represent a new kind of politics."

Voters often say they dislike negative campaigning, but evidence suggests that it works.

John Geer, professor of political science at Vanderbilt University in Tennessee, said: "These attacks can be unpleasant and a cause of anxiety but this exchange is important. It's very informative for voters. The American public needs to know not only if Obama can take a punch but if he can throw one. The image of Obama is that he is cerebral. He needs to show that he has a gut instinct and that he can be a street fighter."

Independent liberal groups such as MoveOn.org have been running TV adverts attacking McCain for months in battleground states.

Last week, days after he joked that being rich in the US meant earning at least $5m a year, McCain acknowledged he wasn't sure how many houses he and his wealthy wife actually own.

The Obama campaign seized on McCain's house gaffe. Speaking on the campaign trail in Virginia on Thursday, Obama mocked McCain for his comments. His campaign followed up with an ad called "Seven", claiming this is the number of houses McCain owns. The ad closes with a shot of the White House and the narration: "Here's one house American can't afford to let John McCain move into."

The Republicans were predictably dismissive of Obama's attacks. Party spokesman Brian Rogers said: "Does a guy who worries about the price of arugula and thinks regular people 'cling' to guns and religion in the face of economic hardship really want to have a debate about who's in touch with regular Americans?"

Nationwide, Obama has spent about $46m on TV advertising in the past two months, compared with McCain's $38m.

Business and labour interests have donated millions to the Democratic and Republican convention (which starts next week) in exchange for premium seats and special access at the showcase events. Many institutions will also hold parties and receptions where politicians can get an earful from lobbyists and their clients.

New, tougher ethics rules aside, conventions remain the only activity where federal candidates can raise unrestricted amounts of money from wealthy donors, unions or corporations – donations typically called "soft money".

Why? Because the recipients are convention host committees, which technically are not political entities.

Top donors are giving $1m or more to these committees, with a few dozen corporations, including AT&T, Coca-Cola, Pfizer and Qwest, contributing to both conventions.

Million-dollar donors to the Denver Host Committee are rewarded with suites at Invesco Field, the football stadium where Obama will deliver his acceptance speech.

Obama is also offering club level seats at Invesco and an invitation to a special reception for $1,000 to his campaign fund.

Dark horses of the presidential race

All eyes may be focused on Barack Obama and John McCain, but they are not the only candidates standing for the US presidency this year. And as Ralph Nader, blamed for losing Al Gore the presidency in 2000, shows, third party or independent candidates can split votes and cause political upheavals even if they have no hope of gaining office. Here are some of the serious and not so serious candidates.

Bob Barr – The Libertarian Party candidate is a former Republican congressman. He wants to abolish tax and take the US out of Nato.

Ralph Nader – The longtime consumer advocate, right, is blamed by many on the left for handing the election to George W Bush in 2000.

Brian Moore – Socialist candidate. Stood for the Senate in Florida in 2006.

Cynthia McKinney – The Green Party presidential candidate and a former congresswoman from Georgia, top.

Chuck Baldwin – The Constitution candidate is a firm believer in the Bible and the US Constitution and a vocal opponent of the UN.

Gene Amondson – The Prohibition Party candidate is a minister, woodcarver and landscape artist.

Charles Jay – The Boston Tea Party. Despite the name, not an American version of the Monster Raving Loony Party: "The Boston Tea Party supports reducing the size, scope and power of government at all levels and on all issues."

Alan Keyes – The American Independent Party. He ran for presidency in 1996, 2000, and 2008, and was a Republican nominee for the US Senate in 1988, 1992, and 2004.

Seth Tyrssen– American Fascist Party. A former actor, Tyrssen believes Fascism has had a bad name.

Diane Templin – The American Party. Diane's website declares: "She just might be the next Maggie Thatcher!"