WASHINGTON — The sharpening rhetoric between John McCain and Barack Obama over their competing plans to overhaul the nation's tax system has underscored one of the most profound differences between them — how they would treat America's wealthiest taxpayers.

Under McCain, the rich would see their tax burden ease. Under Obama, their rates would rise dramatically.

For much of the campaign, the two candidates have talked sparingly and obliquely about how they would deal with affluent taxpayers. But a recent volley of acidic campaign ads stirred up the tax issue, and a forum question posed last weekend by a Southern California Christian church pastor, Rick Warren, zeroed in on how both men defined "rich."

Obama said the dividing line was at $250,000, while McCain responded somewhat flippantly that it was $5 million. McCain aides said later that the senator was joking, but his remark quickly became a campaign flashpoint.

"I guess if you're making $3 million a year, you're middle-class," Obama sniped, prompting a McCain aide to fire back: "It's not the job of the government to define who is rich."

The argument over where to draw the line among the nation's wealthiest taxpayers is the central difference between rival tax blueprints that pose starkly differing formulas for reviving a faltering economy.

A close look at their proposals shows that the candidates' differences fall neatly along the traditional policy gulf that has long divided Republicans and Democrats: liberating the wealthy with tax cuts to stimulate the nation's prosperity, versus raising their rates to redistribute the tax burden and pay for crucial government programs.

"The real fault lines are over how to treat people in the highest tax brackets. It gets to the heart of their economic philosophies," said Leonard E. Burman, a senior fellow with the Tax Policy Center, a nonpartisan Washington-based tax reform group that has questioned the details of both tax plans.

McCain's plan would cater to wealthy taxpayers and corporations by extending and adding to the Bush tax cuts, slashing the corporate tax and weakening the estate tax, but would also aid taxpayers across the board by pressing to make the full Bush cuts permanent.

A deficit hawk and a former critic of the massive tax cuts launched in 2001 by the Bush administration, McCain now embraces the tax policies of supply-side economists who contend that lifting the tax yoke on the rich would encourage investment and stimulate the economy.

Under his proposals, McCain would make all of the 2001 and 2003 Bush tax cuts permanent. That would keep the highest two tax brackets at their present rates of 33 percent and 35 percent.

"They've been law for eight years now, and taxpayers are used to those rates," said J.D. Foster, a senior fellow at the conservative Heritage Foundation in Washington. "Allowing them to expire would be tantamount to a massive tax increase."

McCain also has proposed a sharp reduction in corporate taxes. He would pare the highest two corporate tax brackets, 35 percent and 34 percent, down to 25 percent. The top bracket would be immediately eliminated and the 34 percent bracket would be phased down to 24 percent between 2009 and 2014.

He would also maintain the 15 percent tax rates on dividends and capital gains for the highest-tier taxpayers. And starting in 2010, McCain would substantially reduce the estate tax. He would increase the exemption on inherited funds from $3.5 million to $5 million and sharply lower taxes on remaining wealth from 45 percent to 15 percent, moves that would enable affluent families to hold on to more of their wealth.

Democratic-leaning economists say McCain's plans offer little new aid to squeezed middle-class families. And they question whether corporations and wealthy taxpayers would convert McCain-era tax savings into new investments that would bolster the economy.

"McCain does nothing about income inequality," said John Irons, research and policy director for the Economic Policy Institute, a center-left think tank backed by labor interests. "It's skewed toward upper-income Americans to the exclusion of most everyone else."

Obama's tax plan skews the other way, aimed at strengthening benefits for lower-rung taxpayers and raising rates at the top. His plan would increase rates for the highest two tax brackets to 36 percent and 39.6 percent, restoring those tiers to levels last seen during the Clinton administration.

In Springfield, Mo., last month, Obama pledged that if "you're a family making less than $250,000 a year, you will not see your taxes go up." Obama would exempt seniors making less than $50,000 from paying any income tax. And he would make the Bush cuts permanent for poor and middle-class Americans, adding tax breaks such as a refundable credit for wage earners and a higher education credit for students who agree to work 100 hours of community service.

Under Obama's plan, the highest corporate tax tier would hold at 35 percent — a nod to Wall Street, but higher than McCain's slashed rate. But Obama would raise the highest rates on dividends and capital gains from 15 percent to 20 percent. And he would keep the estate tax in the same form approved by Congress for 2009, with an exemption for the first $3.5 million and a top tax rate of 45 percent. That would mean a higher rate on the wealthy than McCain would allow.

Conservative economists caution that Obama's tax hikes on the wealthy and corporations would increase the drag on the sluggish economy. "It would lead to disincentives for savings and productivity," said Alan D. Viard, a former senior economist for the Federal Reserve Bank in Dallas who is now a resident scholar at the conservative American Enterprise Institute. "Over time, it would mean less capital accumulated and would ultimately force wages lower."