Steve Forbes is filling airwaves with ads that go after George W. Bush on taxes. Bush is bristling back. Orrin Hatch is airing a 28-minute denunciation of President Clinton. Interest groups are taking their agendas to the airwaves, slamming candidates who don't agree with them.

And Al Gore, who promised Wednesday to run a positive campaign, is readying an ad that at least subtly jabs Bill Bradley on farm policy, suggesting the vice president is the lone candidate who cares about farmers.

Voters say they want a positive discussion of issues, and early ads have been mostly positive. But with just 12 days until the first votes are cast, the campaign for president was starting to take a nastier turn.

"When you get behind, you get desperate to try to move the (poll) numbers, and you're willing to try attacks," said Bill Benoit, who studies political advertising at the University of Missouri. "The fact that voters say they don't like negative ads doesn't mean they don't work."

Fearing a backlash, candidates promise to stay positive.

"I don't believe in negative campaigning," Bush told reporters Wednesday.

Just days ago, his campaign was taping footage for a sharply worded ad showing people on the street criticizing John McCain's tax plan. Then Bush and McCain agreed during a debate to forgo negative ads.

Bush's campaign put the ad on the shelf. But a college student who was filmed, Heidi Quigley, wasn't happy about being coached to poke at McCain, and she defected to the Arizona senator's camp.

That left Bush aides pedaling backward, saying they were simply preparing in case McCain went negative first. "There is no ad, so there is no issue," said Mark McKinnon, Bush's media adviser.

Asked about the situation Wednesday, Bush said, "The ad will not run." He added, "I'm going to treat my friend and opponent with respect."

McCain's campaign didn't buy the Bush line. "Taping a negative ad and then promising not to run it is the high-tech version of smoking pot without inhaling," said Dan Schnur, a McCain spokesman.

Another GOP rival, Steve Forbes, is flooding Iowa airwaves with an ad accusing Bush of breaking a Texas promise not to support tax hikes. In the ad, an anti-tax activist accuses Bush of "a record of broken promises" on taxes.

In 1997, Bush supported a tax-cut bill that included some increases to the sales tax, which critics say was a violation of his no-new-taxes pledge, made in 1994.

Bush is responding. In a new ad planned for Iowa, Bush promises to run a campaign free of "cynical and negative politics." He also says, without naming Forbes, that "my opponent has chosen to run a negative campaign," advisers said.

He's hoping Iowa voters will recall how Forbes used a barrage of negative ads to weaken eventual nominee Bob Dole during the 1996 primaries.

The Democratic jousting over the airwaves is more subtle.

Gore is preparing a TV ad for Iowa in which a home state senator, Tom Harkin, calls the vice president the only candidate who really cares about farmers. Like other ads, it will not mention Bradley by name.

But Iowa voters are sure to get the message. In other ads, Gore declares he is the "only Democrat" who will protect Medicaid and Medicare and the "only Democrat" who has made education a priority.

Still, Gore is promising to stay positive.

"I think that people want to know how you can make this a better country, not how you can attack somebody else," he said Wednesday in Iowa.

Bradley's ads make similar, subtle contrasts with Gore. In one, he says that "people" accuse him of offering risky ideas. Those "people" are the vice president and his allies.

In the same ad, Bradley declares the "real risk" is in doing nothing about gun control, child poverty and the uninsured. Bradley says the "do nothing" tag wasn't intended for Gore.

While candidates fear that attacking their opponents will backfire, outside groups regularly do it.

An abortion rights group recently ran ads saying Bush poses a "grave risk" to legal abortion. The Sierra Club has its own ads accusing the Texas governor of doing little for the environment. McCain has been hit in ads by anti-tax activists and anti-abortion groups.

One ad feeds on another, and TV viewers should expect plenty more this year from groups of all political stripes, said Larry Sabato, a political scientist at the University of Virginia.

"It's a matter of keeping up with the Joneses," he said. "If some of your competitor interest groups are airing ads you want to get out there, too."