A gladiatorial President Barack Obama on Tuesday accused Republicans of plotting "social Darwinism" and hammered out the economic vision on which he will build his bid for a second White House term.

Obama, apparently bristling to start mixing it with his Republican foe ahead of November's election, made an impassioned case for his political project of using government to help build a fair US economy for the future.

He rebuked his most likely Republican opponent Mitt Romney by name, calling him to account for describing what Obama sees as a "radical" budget passed by congressional conservatives as "marvelous."

"Instead of moderating their views even slightly, the Republicans running Congress right now have doubled down," Obama said, styling the $3.5 trillion spending plan as a historically extreme right wing document.

"This congressional Republican budget is something different altogether. It is a Trojan horse," Obama said.

"Disguised as deficit reduction plans, it is really an attempt to impose a radical vision on our country. It is thinly veiled social Darwinism.

"It is antithetical to our entire history as a land of opportunity and upward mobility for everybody who's willing to work for it. It is a prescription for decline."

Obama chose to unleash his attack at a meeting of newspaper editors and reporters on the day Romney is expected to march closer to the Republican nomination, in nominating contests in Wisconsin, Maryland and Washington DC.

Romney's campaign, and fellow leading Republicans, hit back hard and fast, reflecting the extent to which the political heat is rising seven months before election day.

"If President Obama is assigning blame for the country's debt and deficits, he should look no further than his own budget blueprints," said Romney campaign spokeswoman Andrea Saul.

"After piling on trillions of dollars in new debt in his first three years in office, the last thing President Obama is qualified to lecture on is responsible federal spending."

Republican House Speaker John Boehner accused Obama of resorting to "distortions and partisan pot-shots."

"Americans continue to be disappointed that the president is shrinking from... challenges rather than displaying the courage needed to solve them."

The Republican budget, framed by Wisconsin congressman Paul Ryan, an Obama rival and intellectual engine of the Republican Party passed the House of Representatives last week, effectively sketching the fiscal battleground ahead of the November 6 election.

It proposes the reform of the Medicare system of health care for elderly Americans, calls for tax reform and, among other measures, cuts the top individual tax rate to 25 percent and lowers corporate taxes.

Republicans say the approach will spur growth, create jobs, cut the deficit, secure health care for future generations of retired Americans and put the US economy on a sustainable fiscal course.

Obama also made a new push for the "Buffett rule" a plan named after billionaire financier Warren Buffett, that would mandate that all Americans earning over $1 million pay at least a 30 percent tax rate.

"I intend to keep fighting for this kind of balance and fairness until the other side starts listening, because I believe this is what the American people want," he said.

Senior White House officials believe that Obama can prevail in a politically divided nation by contrasting his vision of a land where the middle class gets a "fair shot" with Republican policies he says brought on the economic crisis.

A USA Today/Gallup poll released Monday showed Romney slipping further behind the president, who led the former Massachusetts governor 49 to 45 percent among registered voters.

Obama's lead in 12 swing states was even greater, with the president ahead 51-42 on average in key states like Florida, Ohio and Pennsylvania.

The president appears to be benefiting from a brutal Republican primary season, which has dinged Romney, who has been unable to kill off his main remaining opponent Rick Santorum.

Obama may also be reaping the results of an increasingly resilient US economic recovery and accelerating job creation, which has reduced the unemployment rate to 8.3 percent.