Republican presidential candidate George W. Bush, right, shakes hands with Republican rival Steve Forbes during the annual Iowa Farm Bureau Convention in Des Moines Wednesday. (AFP photo / Justin Peterson) DEBATE DETAILS Details of Thursday's debate in New Hampshire among Republican presidential candidates: Who: Gary Bauer; Texas Gov. George W. Bush; Steve Forbes; Sen. Orrin Hatch, R-Utah; Alan Keyes; Sen. John McCain, R-Ariz. Where: WMUR-TV's studio, Manchester, N.H. When: 8 p.m.- 9:30 p.m. EST Coverage: Live, by Fox News Channel and WMUR-TV. Sponsors: Fox News and WMUR-TV. Moderators: Brit Hume, Fox News Washington managing editor, and Karen Brown, WMUR news director and anchor.

DES MOINES, Iowa -- In a flurry of charges and countercharges, the Republican presidential campaign took a turn for the negative Wednesday on the eve of a high-stakes debate.

With the top three candidates previewing their strategies, Steve Forbes aired his first critical advertisements and called George W. Bush's new tax plan "something only the timid could love."

Bush dismissed Forbes with an unusually personal barb: "He likes to campaign by tearing people down."

And Sen. John McCain took a subtle dig at the GOP front-runner by questioning Bush's commitment to banning Internet taxation.

"They're starting to tip their hands about their plans for the debate, and it sounds like it could be a negative affair," said Dean Spiliotes, professor of government at Dartmouth College in New Hampshire.

All six Republican presidential candidates planned to attend Thursday night's debate in Manchester, N.H. It will be the fourth GOP debate, but the first that really matters; Bush did not attend the others.

Each White House hopeful has something to prove.

Conservative activist Gary Bauer, Sen. Orrin Hatch of Utah and former ambassador Alan Keyes need a boost from the back of the pack. The stakes are even higher for Bush, McCain and Forbes.

Bush hopes to show he has what it takes to be president, following a series of foreign policy gaffes and claims by rivals that he is not ready for the job.

"In a funny way, the bar has been lowered so much by Bush's critics that all he needs to do is show up and he'll surpass expectations," said David Carney, a New Hampshire consultant.

The two-term Texas governor delivered a policy address last month, unveiled his tax-cutting package Wednesday and reluctantly agreed to Thursday's debate -- all efforts to shrug the "lightweight" label his rivals want to stick on him.

The five-year, $483 billion tax package targets low- and middle-income workers as well as the rich, underscoring Bush's promise to be a "compassionate conservative."

It also is handy debate fodder for Bush, who has been accused of avoiding detailed policy discussions. "How about this for specifics?" Bush asked reporters with a wink and a grin.

He and Forbes attended a pancake breakfast for farming leaders, a low-key event that surprisingly yielded the sharpest exchange between the two rivals.

As Bush and a horde of journalists made their way to the middle of the dining room, Forbes ushered his much smaller scrum to the same spot. Shaking hands, the foes wished each other luck in the debate.

The truce lasted less than 10 seconds, when Forbes was asked his opinion of Bush's tax-cut plan.

"Something only the timid could love -- simply more political expediency," he replied.

Bush was out of ear shot, but the line was quickly relayed. "He likes to campaign by tearing other people down," Bush said. "That's just not how I'm going to campaign. Republicans will have a choice."

"This is a guy who made his reputation tearing down other people. I'm going to talk about how I intend to make the economy better, how I intend to help people on the outskirts of poverty," Bush said. "That's how I'll campaign. And that's how people in the Republican party ought to campaign."

That brief exchange highlighted strategy shifts by both candidates, their advisers said afterward.

Forbes plans to step up his critique of Bush's record and policies, hoping to improve his weak showing against Bush in polls. Bush, fearing that the millionaire publisher Forbes has the money to make accusations stick, will be more likely to respond to criticism.

The Texan also will try to inoculate himself against attacks by labeling any criticism "negative campaigning."

Bush is relying on poll data that suggest voters are tired of political bickering. Many voters, particularly in Iowa, recall Forbes' blitz of critical ads in 1996 that staggered eventual nominee Bob Dole.

Bush will defend his record and "might find it useful in a case or two to say, `Is this the kind of politics you really want?" according to an adviser, speaking on condition of anonymity.

Forbes recently released two critical ads, one questioning Bush's commitment to retirees and the other challenging his "compassionate conservatism" slogan.

Comparing policies and philosophies is healthy, not negative, Forbes told reporters.

"Governor Bush goes with politics as usual and trimming around the edges. I want to go to the heart of the problem," Forbes said.

One of his advisers, Keith Appell, said Forbes will look for openings to question Bush on taxes, spending, foreign policy, education, health care -- "on everything.'

"Just spin the dial and pick an issue," Appell said, "and Forbes believes Bush is vulnerable."

Forbes may be hindered by a format that doesn't allow much give-and-take between candidates.

McCain isn't expected to be nearly as blunt as Forbes. The Arizona senator has climbed into a tie with Bush in New Hampshire, primarily by rallying the anti-Bush, pro-underdog vote. He stands to benefit if a Forbes goes negative and voters reject Bush and the millionaire conservative.

Asked what he wanted voters to conclude from his first showdown with Bush, McCain said, "In a word, that I'm presidential," McCain replied.