George W. Bush and John McCain stepped up their war of words on the airwaves yesterday, with each GOP presidential candidate criticizing the other in commercials for the first time.

Without mentioning his rivals by name, McCain says in a new ad: "There's one big difference between me and the others--I won't take every last dime of the surplus and spend it on tax cuts that mostly benefit the wealthy. I'll use the bulk of the surplus to secure Social Security far into the future to keep our promise to the greatest generation."

The "benefit the wealthy" charge is a direct shot at Bush, whose $483 billion proposed tax cut would provide substantial benefits for those in the highest income bracket. The Arizona senator is increasingly staking his New Hampshire campaign on the argument that it is more important to shore up Social Security and pay down the national debt, while offering a tax cut half the size of Bush's.

The Texas governor, for his part, is explicitly playing defense in his latest ads after months of crafting his own message.

In a spot prepared for Iowa, he addresses criticism from McCain and Steve Forbes. "One of my opponents says my tax cut for America is too big, too bold," Bush says. "Another has raised questions about my record. They're both wrong. In Texas, you're only as good as your word. In 1997, I cut taxes more than $1 billion. In '99, I cut taxes nearly two billion more."

Bush is responding to a Forbes ad charging that he broke his vow not to raise taxes. The governor made an unsuccessful proposal to boost some state sales taxes that was more than offset by other tax reductions.

Bush also takes aim at McCain in a New Hampshire ad, saying: "The public is going to have to choose between someone who wants to have a realistic tax-cut plan . . . or are we willing to take the risk to leave money in Washington. Now, money left in Washington is likely to be spent on bigger government, and that's the fundamental difference."

Stuart Stevens, a Bush media adviser, said the campaign is responding to Forbes because "it's worth setting the record straight, particularly on a subject we want to talk about anyway." He said McCain "spent the fall trying to show how much he agrees with Bill Bradley on campaign finance reform, and seems determined to spend the winter showing how much he agrees with Al Gore on taxes. It is class warfare."

McCain's communications director, Dan Schnur, responded that "the only people who think this has anything to do with pitting one income group against another are the Bush campaign staff. We're confident in this message; otherwise we wouldn't have put it in an ad."

The candidates continued to take potshots on the campaign trail yesterday. At a hastily arranged news conference at the Des Moines airport, Bush tried to paint McCain as a Democratic sympathizer, noting that "the two voices criticizing my plan are Al Gore and John McCain." In New Hampshire, McCain told reporters that "I think it's conservative to pay down the debt and save Social Security" rather than indulge in a more sweeping tax cut.

Bush also accused McCain of engaging in "Washington double-accounting" by using his proposed savings from eliminating Iowa's popular ethanol subsidies to finance his tax cut and a school voucher program. But Schnur said McCain does not count the savings for tax relief and that "Governor Bush has yet to propose the closure of a single tax loophole or the elimination of a single spending program."

Bush's financial advantage--he has raised more than $67 million to McCain's $21 million--is enabling him to advertise in nine states to McCain's three. Aides say Bush will remain on the air in Iowa, New Hampshire, South Carolina, Arizona, Michigan, North Dakota, Virginia, Washington and Delaware until those states vote.

CAPTION: In ads, Sen. John McCain (Ariz.) stresses need to shore up Social Security while Gov. George W. Bush (Tex.) answers foes' criticism.