Texas Gov. George W. Bush and Arizona Sen. John McCain clashed here tonight over tax cuts, ethanol subsidies and campaign finance reform, with Bush charging that McCain's proposals to rein in campaign contributions would "hurt the Republican Party" and help the Democrats.

The exchange during a 90-minute debate among the six Republican presidential candidates reflected the intensifying competition between Bush and McCain for the Republican nomination. It also marked the first time in three candidate debates that the two candidates, who have gone out of their way to be nice to each other, tangled over policy and disagreed so openly.

Bush came under attack from most of his rivals tonight on issues ranging from abortion to China and the World Trade Organization. But in contrast to previous debates, Bush appeared more aggressive in presenting his views, casting himself as a candidate not from Washington and as a man with strong conservative convictions.

Asked to name the philosopher who had affected him most, a question tossed to all the candidates, Bush responded, "Christ, because he changed my heart."

Tonight's debate, sponsored by WHO-TV and carried nationally on MSNBC, likely set the tone for the next five-plus weeks of campaigning before the Jan. 24 precinct caucuses that will begin the nomination process. The other participants included magazine publisher Steve Forbes, former Reagan administration official Gary Bauer, former ambassador Alan Keyes and Utah Sen. Orrin G. Hatch.

Forbes is staking his candidacy on a strong second-place finish in Iowa but was challenged tonight by Bauer, who along with Keyes is competing for support among religious and cultural conservatives, who play an influential role in the GOP caucuses. Bush arrived as the clear front-runner in Iowa but under pressure after two recent debates in which he was judged to have given lackluster performances.

McCain is not formally competing in the Iowa caucuses, having chosen to concentrate his time in New Hampshire, where he has pulled ahead of Bush in recent polls. But he closed the debate tonight by asking that "a hardy . . . band of brave souls" in Iowa turn out and vote for him here, a sign of his desire for a surprise finish to boost his candidacy in New Hampshire.

Early in the debate, displaying his penchant for challenging otherwise friendly audiences, McCain went on the offensive by directly attacking one of the favored subsides of Iowa farmers, the subsidy for ethanol production.

"I'm going to tell you the things that you don't want to hear as well as the things you want to hear," McCain told the audience of Republican activists. "Ethanol is not worth it." He added that everyone else on the stage would share his view "if it weren't for the fact that Iowa is the first caucus state."

As scattered boos filled the Des Moines Civic Center, other candidates jumped in to defend the subsidies. When Bush got his turn, he said, "I'd have supported ethanol whether I was here in Iowa or not."

Their exchange over taxes began when Bush challenged McCain for criticizing the governor's $483 billion tax cut proposal as excessive. McCain's plan, Bush charged, did not do enough to help struggling single parents making $40,000 or less who are trying to break into the middle class.

"That single mom with . . . two children gets no tax cut" in McCain's plan, Bush said.

McCain didn't directly answer the question and quickly turned the debate to reforming the campaign finance system, the heart of his campaign agenda. McCain then urged Bush to join in pledging to eliminate "soft money"--the large, unregulated contributions by individuals, labor unions and corporations that go to the two political parties.

"You and I can stop that tonight," McCain said. "We can commit as nominees of the party that we will have nothing to do with soft money. . . . We can get the special interest money out of American politics."

Bush said he also opposes unlimited labor and corporate contributions but said McCain's approach amounted to "unilateral disarmament" by the Republicans unless it was accompanied by legislation that would prevent labor unions from using union dues for political purposes without the express approval of their members.

"Here's my worry with your plan," Bush said. "It's going to hurt the Republican Party."

After the debate, Bush said others would have to decide whether McCain had answered his tax question about helping workers with income under $40,000. McCain, pressed in a post-debate news conference, said he would expand to the 15 percent tax bracket to workers making up to $70,000. He also said he would consider expanding the Earned Income Tax Credit and opposed taxing commerce on the Internet.

Bauer repeatedly pressed Bush on abortion and on China. In the segment of the debate that allowed candidates to question one another, Bauer challenged Bush to commit to choosing a vice presidential running mate who opposes abortion.

Bush, who opposes abortion, refused. "I'll name somebody who can be the president," he said. "That ought to be the main criterion."

Bauer also said politicians in Washington had allowed "China to play us for suckers" by refusing to open their markets to U.S. commodities. But Bush challenged Bauer for opposing China's entry into the World Trade Organization, arguing that it would mean increased corn exports for U.S. farmers.

McCain and Hatch also spoke up tonight in favor of opening up trade with China, which caused Bauer at one point to accuse the others of "naivete about Chinese leaders that is breathtaking."

Throughout the debate, the candidates appeared to be trying to get to the right of one another, reflecting the conservative cast of the Iowa GOP electorate.

An opening question about last spring's shootings in Littleton, Colo., quickly turned to answers by Keyes and Bauer about protecting the unborn and eliminating abortion. Keyes defended the family as one of the "bedrock sources of the moral character of this nation."

And on the question about influential philosophers, Bauer and Hatch also cited Jesus Christ, as did Bush.

Bauer, Forbes and Keyes in particular appeared to be competing with one another for the allegiance of those state's most conservative voters. In his closing statement, Bauer cast aspersions at Forbes's conservative credentials, saying that he was raising money "for crisis pregnancy centers while my friend Steve Forbes was raising money for [New Jersey Gov.] Christie Todd Whitman," whose support for abortion rights has made her anathema to the right.

The exchange that drew the most laughter from the audience came when Hatch, preparing to question Forbes, told his rival he was about to toss him a home run ball. "That usually means hold your wallet," Forbes quipped, to which Hatch replied, "Steve, I couldn't even lift your wallet. I'm running the skinny-cat campaign."

CAPTION: Republican presidential hopefuls Alan Keyes, left, and Texas Gov. George W. Bush share a light moment during the GOP debate in Iowa.