United States Senator Barack Obama is to make a 30-minute TV prime time advertising blitz on Wednesday.
The advertisement for the Democratic presidential candidate will be broadcast on three of the four national networks: CBS, NBC and Fox.
The slots at 8pm on the east coast and 7pm in the central zone, precede what could be the deciding game in baseball's World Series on Fox.
At Fox's request, Major League Baseball has agreed to delay the start of the game - which could decide the seven-game series - to fit in the commercial.
Senator Obama is ahead of Republican John McCain in the polls a week before election day.
The Obama campaign would not disclose the contents of the commercials, which are the first of that length to feature in a presidential campaign since billionaire Ross Perot's independent candidacy in 1992.
The Campaign Media Analysis Group calculates that the air time would cost about $US5 million. USA Today estimates the bill to be about $US3 million.
Senator Obama has raised record sums from private donors. His total spending on campaign advertising is forecast by some to reach $US250 million - more than the advertising budgets of big retail groups like Burger King, Apple and Gap.
Senator Obama is also appearing afterwards on a popular late-night comedy programme, The Daily Show.
The election will be held on 4 November.