The president is pushing for the so-called Buffett rule, named after billionaire investor Warren Buffett, an Obama supporter who has called for tax hikes on the wealthy after famously observing last year that his secretary pays tax at a higher rate than he does.
"That's wrong; that's not fair," Obama said at a campaign-style event in Florida that saw him take direct aim at congressional Republicans for their refusal to back legislation that would essentially see millionaires pay at least 30 per cent of their income in tax — about the same as the middle class.
"We want folks to get rich in this country; I think it's wonderful when people are successful — that's part of the American dream," he said. "But understand that the share of our national income going to the top one per cent has climbed to levels we haven't seen since the 1920s."
The richest people in the country are also paying taxes at one of the lowest rates in 50 years, he added.
"You shouldn't get special tax breaks, you shouldn't be able to get special loopholes," he said to cheers from a college audience in Boca Raton, Fla.
Obama also mocked so-called trickle down economics.
"In this country, prosperity has never trickled down from the wealthy few," Obama said. "Prosperity has always come from the bottom up, from a strong and growing middle class."
The U.S. Senate is scheduled to hold a procedural vote on extending debate on the Buffett rule next Monday, but the measure isn't expected to become law.
Nonetheless the Buffett rule has become a centrepiece of Obama's re-election campaign, aimed in part at drawing a stark contrast between a populist president and Mitt Romney, the multimillionaire Republican presidential front-runner frequently accused of being out of touch with everyday Americans because of his vast wealth.
Romney, indeed, is vulnerable on the tax issue. In January, he released his 2010 tax return and a 2011 estimate that showed he paid an effective tax rate of 14 per cent on total income of US$45 million.
He's estimated to be worth as much as $250 million.
Republicans are fighting back, calling the Buffett rule a "gimmick" while accusing Obama of waging class warfare.
"I don't think Americans are interested in class warfare, number one, but number two, this isn't just about millionaires and billionaires," Reince Priebus, chairman of the Republican National Committee, said in a conference call Tuesday just a few hours before Obama's Florida speech.
"This is really about, more importantly, small business owners. Most small businesses file as individuals, and so this is a tax that is going to hurt small businesses at a time when we should be making it easier for people to do business."
What's more, Priebus added, Obama was attacking the American dream — that hallowed notion in the United States that great financial successes can result from hard work, ingenuity and determination.
"In most cases, people in this country want to be able to believe ... (that) we're still a country that if you work hard and play by the rules, you can live the American dream."
A Washington Post/ABC News poll released Tuesday, however, suggested Obama may have a winning argument on his hands.
More than half of the survey's respondents — 52 per cent — said they believed that "unfairness in the economic system that favours the wealthy" is a bigger problem than "over-regulation of the free market that interferes with growth and prosperity." Only 37 per cent felt over-regulation was the bigger problem.
On the offensive on the issue, Obama's campaign website now features a 30-second clip of late president Ronald Reagan, a conservative icon, calling it "crazy" that millionaires paid less in tax than a bus driver did.
The campaign is also taking aim at Romney, accusing the former Massachusetts governor of wanting to "give $5 trillion in tax cuts slanted toward millionaires and billionaires while gutting Social Security and Medicare."
Ahead of Obama's Florida jaunt, the White House also released a report late Monday advocating the Buffett rule.
"Of the 400 highest-income earning Americans, one in three of those 400 paid less than 15 per cent of their income in taxes in 2008," Alan Krueger, the head of the White House Council of Economic Advisers, told reporters.
"If you look at the 400 as a whole, on average they paid just 18 per cent of their income in federal income taxes."
The proposed tax hikes on millionaires would raise an estimated US$47 billion over 10 years, or less than $5 billion a year. The U.S. deficit, by contrast, was $1.3 trillion for the 2011 fiscal year, which ended last Sept. 30.
Rep. Paul Ryan, the architect of Republican budget proposals, scoffed at the purported benefits of the Buffett rule in an appearance on CNBC on Tuesday, saying it will do next to nothing to slash the deficit and rein in the ballooning U.S. debt.
"It pays for about six per cent of the president's proposed deficit spending. He's not using the Buffett rule to close deficits. He's using it to spend more money," Ryan said.
"So it's: 'Take more money from job creators to spend in Washington.' If that was the secret to economic success, we would have learned it by now."
The White House, meantime, says $47 billion over 10 years is hardly insignificant.
"We think $47 billion is actually meaningful and to most Americans would sound very large," said Jason Furman, the deputy director of the White House's National Economic Council.