Confronted with an unexpectedly strong challenge from Arizona Sen. John McCain, George W. Bush has narrowed his message almost exclusively to the need for a broad-based tax cut, an issue that appeals to the Republican Party activists who tend to vote in primaries and caucuses.

Eight months after he first announced his presidential candidacy, the Texas governor has scrapped a standard stump speech that touched on a variety of themes in favor of a more spare talk that trumpets his $483 billion tax-cutting plan and, to a lesser extent, education, trade and defense.

On the campaign trail this week, Bush has portrayed his tax plan as a boon for the working poor and middle class and attacked McCain's tax proposals at every opportunity. During campaign stops today here and in New Hampshire, Bush appeared with families he said would benefit from his plan if he's elected president.

Even though most polls show tax cuts way down on the list of priorities for most Americans, it is still very much an issue for the conservatives most likely to vote in primaries and caucuses, particularly in states such as New Hampshire and Iowa. While the Bush campaign is extremely sensitive to any suggestion that he tailors his message to political considerations, Bush himself suggested today that there's no contradiction between his heartfelt beliefs and sound campaign strategy.

Referring to his tax plan, Bush said: "I'm going to defend it. And I'm going to campaign on it. It's a winning issue for me."

Bush's aggressive demeanor and sharp message on the stump in recent days have been a marked departure in style from the ploddingly efficient campaign he ran most of last year. After announcing his candidacy in June, Bush waited six months to outline his tax policy on Dec. 1 in Iowa. Until then, he typically limited his comments on taxes and economic policy to a line in his stump speech, which was usually some variation of: "It's conservative to cut taxes. It's compassionate to give people more of their money back."

Although Bush has always portrayed himself as an ardent tax-cutter, he chose to focus on other issues in an attempt not only to define himself but also to soften the GOP's image as a party concerned only with the cold, hard numbers of economic policy, campaign aides said.

Yet when asked this week what Bush would be talking about over the next few weeks, campaign spokesman Ari Fleischer responded: "Taxes, taxes, taxes." For his part, Bush has been talking all week about lowering tax rates to spur economic growth and reciting numbers galore.

One Bush adviser, speaking on the condition of anonymity, listed three reasons for the change. "The reason taxes are important is because it is the most important point of distinction between us and Senator John McCain," the adviser said. "Second, taxes are still important in New Hampshire, which as you know doesn't have an income tax. Third, taxes are still a uniting issue for Republicans."

McCain has proposed his own tax cut plan, including expanding the 15 percent bottom tax bracket, but his proposals are collectively smaller than Bush's. McCain will elaborate on his tax plan next week, and aides said it will be geared more to people who are not wealthy.

Bush has prided himself on running a positive campaign and has tried so hard to stick to that promise that he has mostly refused to be critical of the policies of his rivals, particularly McCain, whom he has gone out of his way to compliment. But with McCain up or even with Bush in the polls in New Hampshire and gaining elsewhere, Bush has suddenly shown an eagerness to engage him on their differences over campaign finance and taxes.

Bush went after McCain again today, denigrating his idea to expand the 15 percent tax--the lowest bracket--to higher incomes. "I think Sen. McCain has got his plan out there and it expands the 15 percent bracket, which means it does nothing for the 70 percent of taxpayers who are already in the 15 percent tax bracket." And he lobbed what in GOP circles is nothing less than a clear insult: He said McCain sounded a lot like Vice President Gore on the tax issue and suggested both men had a "Washington mind-set."

That's mild, perhaps, compared with the daily barrage of criticism that Gore has lobbed at his Democratic rival, Bill Bradley. But for Bush, it's an obvious change.

Eric Wilson, a Bush adviser in Iowa, said today that he has heard some complaining from voters that Bush was not speaking enough on core conservative issues, such as taxes. "To the extent that I'm hearing something, I pass it along," Wilson said. "The most prevalent thing I was hearing was questions about the tax issue."

Some of the people at Bush's campaign event here this morning said they appreciated his focus on taxes and said prior to today they had no idea where he stood.

"The taxes, that was the news to me," said Meredith Johnson, a 71-year-old bus driver. "I just got married last year and I got hit by that marriage penalty. So I was glad to hear him talk about that." Bush has proposed lessening the penalty.

Bush adviser Ralph Reed said Bush's strategy is already showing dividends, noting that a new New Hampshire poll has Bush even with McCain, after a poll a few weeks ago gave McCain a nine-point edge.

"The feistiness Governor Bush is now showing makes it clear that he's not going to take this race for granted," Reed said. "This is not a front-runner trying to coast to the finish line."