US President Barack Obama will step up his reelection campaign Tuesday by accusing Republicans of pursuing an economic policy akin to ruthless "social Darwinism" that is a prescription for decline.

On the day Mitt Romney, his likely rival in November, is expected to march closer to the Republican nomination, Obama will unleash a searing critique of what he sees as the "radical" fiscal philosophy of congressional conservatives.

He will roast a budget passed by the Republican House of Representatives, and backed by the party's presidential candidates, as a "Trojan horse," according to excerpts of a major speech he was due to deliver later Tuesday.

"Disguised as deficit reduction plan, it's really an attempt to impose a radical vision on our country. It's nothing but thinly-veiled Social Darwinism," Obama was to say at a luncheon sponsored by the Associated Press.

The budget, framed by Wisconsin congressman Paul Ryan, an Obama rival and intellectual engine of the Republican Party, effectively sketched the fiscal battleground ahead of the November 6 election.

The Ryan budget 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.

But Obama argues that the plan will punish the middle class and further enrich the wealthy, a message around which he is building his campaign for a second White House term.

"It's antithetical to our entire history as a land of opportunity and upward mobility for everyone who's willing to work for it," Obama was to say in his speech, in a repudiation of trickle-down economics.

"By gutting the very things we need to grow an economy that's built to last -- education and training; research and development -- it's a prescription for decline."

Republican voters head to the polls on Tuesday in Wisconsin, Maryland and Washington, DC in the latest set of nominating contests expected to further solidify Romney's stranglehold on the party's presidential ticket.

Though Obama is yet to fully spring into campaign mode, there are unmistakable signs that his Chicago-based election team is stepping up the pace of attacks, especially on Romney.

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.

The president will warn on Tuesday, however, that although the economic climate is improving, too many people are still feeling the lash of the deepest economic recession since the 1930s Great Depression.

"Whoever he may be, the next president will inherit an economy that is recovering, but not yet recovered, from the worst economic calamity since the Great Depression," Obama will say, according to the excerpts.

"Too many Americans will still be looking for a job that pays enough to cover their bills or their mortgage.

"Too many of our citizens will still lack the sort of financial security that started slipping away years before the recession hit."

Obama will also use his speech on Tuesday to make 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.