Barack Obama will try to close the deal with American voters tonight with an extraordinary half-hour, prime-time television address – the first advertisement of this duration to be broadcast by a presidential candidate since the early 1990s.

The Democratic candidate can afford to make his ambitious pitch because he has already surpassed all expectations by raising more than $600m (£380m). He is spending the money in an attempt to reach the 25 per cent of US voters who are registered as independent.

His "infomercial" reflects the increasing assurance of the Illinois senator in the final days of the campaign. The content has been kept under lock and key, it will air on three networks and will cost an estimated $1m per network. Fox, CBS and NBC have all cleared their 8pm schedules to broadcast it. Mr Obama has used his cash and regular campaign visits to target Republican-voting states such as Montana and has broken through in states which previous Democratic candidates ignored.

Download the new Independent Premium app Sharing the full story, not just the headlines

There is a plenty of hand-wringing about the excessive influence of money in US elections, but not, for a change, among the Democrats. Although they have long complained about Republican access to corporate money, they are staying quiet this time and are in no mood to surrender the huge advantage they have developed.

Any attempt at reform is also complicated by loopholes that allow well-funded single-issue groups to influence elections through so-called "527 groups", which can accept unlimited contributions for political advertising.

The Obama campaign's secret weapon has been the perfection of micro-donations which have come from more than three million supporters. The effective use of online fund-raising has helped Mr Obama tear up the rule book under which candidates for both parties used public finances while shaking down corporate America and wealthy donors to fund their campaigns.

The Democrat's campaign is now so flush with money that advertisements are all but blotting out the light for John McCain, whose campaign had to accept $85m in taxpayers' money to continue to make his case to the electorate. He has tried to make up for the lack of funds by putting himself and his running mate Sarah Palin on national television as often as possible, even when it has meant that she was lampooned as out of her depth.

Mr McCain's cash shortfall has been more than compensated for with money raised by the Republican National Committee, which finished last month with $77m in the bank. But he has still been outspent by three-and-a-half-to-one on television, according to the Campaign Media Analysis Group.

Mr Obama has steered clear of public finances, which has enabled him to keep raising phenomenal amounts of money throughout his campaign. His campaign manager David Plouffe recently send a video message to supporters revealing that 632,000 new donors signed up in September, bringing the total to 3.1 million.

And the campaign has been spending money as fast as it has hit his account. He is believed to have shelled out $250m on television over the past five months, and has already dwarfed the $188m that George Bush spent getting re-elected in 2004.