WASHINGTON — Senator Obama is going for the gold.

The Democratic presidential contender has decided to buy $5 million in national advertising on NBC during the broadcast of the Olympic games, an NBC spokeswoman confirmed yesterday. The ads will appear on network and cable channels.

The ad purchase was first reported yesterday on the Web site of Advertising Age, a magazine that covers the industry.

The Olympics, to be held in Beijing, will open Aug. 8. Such an extensive purchase of ad time would give Mr. Obama wide exposure before the Democratic National Convention, to be held the last week in August.

Mr. Obama has the resources: He has set fundraising records and reported raising $52 million in June, more than twice the $21.5 million raised by his rival, Senator McCain.

At the same time, the expenditure is as significant for its reach as for the bold statement it makes. Mr. Obama has been airing ads in 18 states, reaching key battlegrounds as well as states that have traditionally voted Republican in presidential elections, including Alaska and Virginia.

A national network ad is highly unusual in politics, because of its expense and because it reaches audiences that are not necessarily targeted by a campaign.

In early February, Mr. Obama, then neck-and-neck with Senator Clinton heading into the Super Tuesday primaries, aired ads during the Super Bowl in television markets serving 24 states that were in play on Feb. 5 and beyond.

The Obama campaign would not comment on the purchase.