The presidential campaign is hitting the airwaves in earnest this week, with Bill Bradley rolling out his first commercials of the season and Steve Forbes abandoning plans to fire negative ads at GOP front-runner George W. Bush in favor of serious issue spots.

In the latest sign of an accelerated political season, all the major contenders will be on the air in Iowa or New Hampshire 2 1/2 months before the voting begins. At the same time, Bush and John McCain yesterday opened a new advertising beachhead in the early primary state of South Carolina.

These moves and counter-moves underscore the kind of crucial questions the candidates face in deciding when and where to spend their cash on television image-making. The risk of an early advertising blitz is that it could deplete a campaign's coffers before the stretch run, when far more voters are paying attention.

But the decision by McCain and now Bradley to enter the advertising fray illustrates the fierce pressures of the 2000 race, in which even candidates whose strategy depends on conserving their resources feel compelled to get on the air early.

Bradley, for example, waited one month after Vice President Gore launched the first of four commercials before announcing yesterday that he would no longer cede the airwaves to his Democratic rival. Bradley's first two spots, which are biographical in nature and were produced by a team of Madison Avenue executives, premiere in Iowa and New Hampshire tomorrow.

"We've never let the Gore campaign set our strategy," said Gina Glantz, Bradley's campaign manager. "We've looked at our budget and the [spending] caps and the pacing of the television. By starting now, we're looking forward to a strong, consistent presence in Iowa and New Hampshire."

While Bush and Forbes, who are not accepting federal funding, can spend unlimited amounts, federal campaign law imposes a more complicated set of spending choices on McCain, Gore and Bradley.

In exchange for about $13 million in public funds for the primaries, these candidates must abide by an overall spending limit of about $40 million, along with tighter state-by-state limits. In New Hampshire, that limit is just $660,000; in Iowa the spending ceiling is $1.1 million.

There are legal ways of circumventing the caps, however. For example, McCain is reaching New Hampshire voters by buying time on Boston TV stations, since only 15 percent of the cost is counted against the Granite State limits. The McCain and Bradley camps estimate that such exceptions mean the effective spending ceiling in New Hampshire is from $2.5 million to $5 million.

The Republican campaigns have been waiting for months to see whether Forbes would deploy his personal fortune to wound Bush with negative advertising, much as he did to Robert J. Dole four years ago. The Forbes camp had already produced ads in which the candidate said he doesn't know where the Texas governor stands on taxes, Social Security and other issues.

But Forbes strategists abruptly dropped plans to air these commercials after showing them to focus groups in Iowa and New Hampshire and concluding that voters would be more receptive to positive, issue-based ads.

Bill Dal Col, Forbes's campaign manager, said the ads implicitly "juxtapose Steve Forbes as a candidate who's discussing issues versus Bush, who's not." He said there was no need to air the anti-Bush spots taped by the campaign because in focus groups "the voters clearly drew those contrasts when they saw the ads."

Forbes's three spots, airing in Iowa and New Hampshire and on national cable networks, include a biographical ad that calls him "a champion of economic growth and a visionary." The others, shot in black and white, show Forbes talking to a group of voters. In one, he calls for future retirees to have the choice of privately investing most of their Social Security funds. The other recycles Forbes's principal theme from the 1996 campaign, calling for a flat tax of 17 percent on all income above $36,000.

Bush, for his part, is launching his first commercial on defense after unveiling several ads on such domestic issues as tax cuts and education. It's no coincidence that the spot is airing in South Carolina, a state with a high proportion of veterans and one of two early-contest states targeted by Arizona Sen. McCain, a former Vietnam prisoner of war.

"Today we live in a world of terror and madmen and missiles," Bush says in the ad. "And our military is challenged by aging weapons and low morale. . . . I will rebuild our military. I will move quickly to defend our country and allies against blackmail by building missile defense systems."

Stuart Stevens, a Bush media adviser, denied that the ad was a response either to McCain's rise in the polls or recent criticism of the governor as lacking foreign policy credentials. "We've wanted to roll this spot out from the get-go," he said. McCain is airing the same "war hero" spot in South Carolina that has been running in New Hampshire--minus the unauthorized footage of the senator at Arlington National Cemetery, for which he has apologized.

The mid-November blizzard of ads allows the candidates to try to connect with voters before much of the country tunes out for the holidays. Scott Reed, who ran Dole's 1996 campaign, said the GOP contenders are also trying to make their mark before the first debate involving Bush, on Dec. 2 in New Hampshire. "Everybody wants to be on the air right now, anticipating a lot of polling that will be done for that event," he said.

The Democratic candidates have made their own calculations. Anita Dunn, Bradley's communications director, said that Bradley's decision to delay advertising until now will leave him with a "significant" financial advantage later on. One Gore ally acknowledged that campaign officials decided last month "to go on the air early even though they believed they would have less money to spend than Bradley" next year.

The vice president's camp argues that its early spending has helped boost him in recent polls. "Al Gore will have all the resources he needs in Iowa and New Hampshire . . . and beyond," said spokesman Chris Lehane.

Despite Forbes's decision to stay positive for now, a moderate Republican group began airing a preemptive ad in Iowa yesterday, urging him to refrain from a negative campaign. "Somebody needs to tell that Steve Forbes that if he doesn't have anything nice to say, don't say anything at all," says the Republican Leadership Council (RLC) ad.

Forbes campaign chairman J. Kenneth Blackwell demanded that the ad be pulled, accusing the Bush campaign of using "its liberal surrogates" to attack Forbes. The Forbes camp charged that 28 of 35 RLC officials are leading Bush donors or supporters. Democratic National Chairman Joe Andrew said it appears that the Bush camp is "using third-party groups to run ads."

RLC executive director Mark Miller said that the $100,000 ad buy had nothing to do with the Bush campaign and that he did not discuss it with board members who back Bush. "We're running this because Forbes has a track record of running a slash-and-burn negative campaign," Miller said.

CAPTION: Sen. John McCain (R-Ariz.) has launched TV ads in South Carolina for his presidential campaign.