PHILADELPHIA  Senator Barack Obama is days away from breaking the advertising spending record set by President Bush in the general election four years ago, having unleashed an advertising campaign of a scale and complexity unrivaled in the television era.

With advertisements running repeatedly day and night, on local stations and on the major broadcast networks, on niche cable networks and even on video games and his own dedicated satellite channels, Mr. Obama is now outadvertising Senator John McCain nationwide by a ratio of at least four to one, according to CMAG, a service that monitors political advertising. That difference is even larger in several closely contested states.

The huge gap has been made possible by Mr. Obama’s decision to opt out of the federal campaign finance system, which gives presidential nominees $84 million in public money and prohibits them from spending any amount above that from their party convention to Election Day. Mr. McCain is participating in the system. Mr. Obama, who at one point promised to participate in it as well, is expected to announce in the next few days that he raised more than $100 million in September, a figure that would shatter fund-raising records.

“This is uncharted territory,” said Kenneth M. Goldstein, the director of the Advertising Project at the University of Wisconsin. “We’ve certainly seen heavy advertising battles before. But we’ve never seen in a presidential race one side having such a lopsided advantage.”