Mayor Bloomberg’s campaign is pouring millions of dollars into television advertisements, promoting his record, sharing his goals for the future – and demolishing his Democratic rivals in the image war. His challengers are nowhere to be seen on TV, except in sporadic free appearances on local news programs, and face a major challenge even once they do start advertising.

The four Democrats, who all are participating in the public campaign-financing system, face a $5.7 million spending cap for the September 13 primary. Mr. Bloomberg, a multibillionaire who is financing his own campaign and thus can spend as much as he wishes, has spent roughly $1 million a week on the air for the past six weeks, and he shows no signs of slowing down an ad campaign that has included new spots almost weekly.

Most ominous for the Democrats, Mr. Bloomberg’s mere presence in this year’s campaign could drive up the price of ads for his rivals. In the 45 days preceding a primary election and 60 days preceding a general election, Federal Communications Commission rules require broadcast stations to offer candidates “the lowest unit charge of the station,” “defined as “no more per unit than the station charges its most favored commercial advertisers.”

But there’s a catch.

The cheapest “class” of ads is the “immediately preemptible” rate. The most expensive is known as “nonpreemptible.” While political candidates generally buy the cheapest class of ads at the cheapest rate, that might not be a prudent choice in this campaign, because preemptible ads, as their name implies, can be thrown out in favor of another, more valuable, offer for the same time slot.

Normally, that’s not a problem because all candidates face the same spending limits and are unlikely to outbid each other. If the New York City Democrats buy ads at the most cost-effective rate, however, they risk having the mayor outbid them and snatch up their TV time.

“It’s sort of a dog chasing its tail when you’re trying to compete with somebody who will out-advertise you 10-to-1, and by that fact alone will increase your same costs for the same impact,” Mr. Bloomberg’s 2001 Democratic opponent, Mark Green, said.

“All we could do is spend what we had. I couldn’t print more money,” he said. “It was frustrating because I had spent so much effort raising x amount of money and now, because of these circumstances, we were only able to buy half x in ads because of the increased ad rates.”

The eventual Democratic nominee will not have a spending cap in the fall campaign, because Mr. Bloomberg’s campaign is self-financed. But the Democratic nominee also won’t have access to a huge personal fortune.

So far, the Democrats of 2005 said they’re not daunted by the challenge.

“We are confident that we’ll have the resources needed to communicate Gifford’s message of leading New York in a different direction than where the mayor is taking us,” a spokesman for Gifford Miller’s campaign, Reginald Johnson, said.

The campaign spokesman for Rep. Anthony Weiner, Anson Kaye, said the congressman has been running and winning campaigns for a long time and has a formula that works, regardless of his opponent’s identity.

“The mayor’s money is his most formidable weapon, but money can’t convince people they like you when they know they don’t,” Mr. Kaye said. “The mayor’s primary asset is money. The Weiner campaign’s primary asset is Weiner.”

The Democrats said, however, that Mr. Bloomberg’s presence might force them to rely less on television advertising and more on alternative techniques of reaching out to voters, such as targeted mailings and direct contact with constituents.

Mr. Kaye said Mr. Weiner would go out to neighborhoods in all five boroughs to communicate directly with New Yorkers. The Weiner team will also conduct a “vigorous mailing campaign” and “explore other media outlets” in its quest to reach the public, he said.

The campaign consultant for C. Virginia Fields, Joseph Mercurio, said Democrats could combat Mr. Bloomberg by being “cognizant of targeting” when they talk to voters.

“We’re going to have to spend a substantial amount of time on targeting to get the most out of the money available,” the Manhattan borough president’s political adviser said. “Obviously, you’re going to have to operate smarter.”

That, he said, meant not conducting what he called a “traditional” campaign like the one Mr. Green led four years ago.

“They bought late. They didn’t pay enough attention to demographics,” Mr. Mercurio said. “He didn’t do enough on dealing with get-out-the-vote activities.”

In its television advertising and other forms of voter outreach, he said the Fields campaign would use the Internet to conduct fund-raising, and to target individual voters and groups, with an “intensity” not seen in previous campaigns.

Mr. Mercurio said it might be that a campaign will be able to afford only one ad on “Law & Order” instead of three, but he said it could adjust to buy ads at other times and on other channels, including cable channels, that targeted narrower but important audiences.

A spokesman for the Bloomberg campaign, Stuart Loeser, said mayoral candidates need to refocus their approach to advertising. “Instead of whining about paying for their ads, they ought to be worried about what they are going to say in those ads since none of them have a single thing to say about how they are going to keep New York moving forward,” Mr. Loeser said.

Although Mr. Bloomberg’s 2005 Democratic opponents said they are not daunted by the advertising challenge posed by a self-financed billionaire, campaign observers said it will be tough for them to reach voters when airwaves are packed with Bloomberg campaign ads and when prices will allow Democrats get less bang for their buck.

A professor of public affairs at Baruch College, David Birdsell, said the Democratic Party and the unions would come to the Democrats’ aid, helping with get-out-the-vote drives and other voter outreach, while Mr. Bloomberg will have to create a network from scratch. As for saturation advertising, however, Mr. Birdsell said, “It’s going to be very difficult, if not impossible, for any of his competitors to compete with him.”

He acknowledged that advertising is a “very shotgun approach” to getting voters, because few of the people who see a television ad will be convinced by it to vote one way or the other, or to turn out at all, on Election Day.

“The question is basically a small percentage of a large number or a large percentage of a small number, and what gives the biggest turnout at the end of the day,” Mr. Birdsell said. “If the Democrats are going to make this work, their structures have to work seamlessly.”