U.S. President Barack Obama autographs posters made by children during a fund-raising stop in Palm Beach Gardens, Fla., on April 10, 2012, before making his speech. REUTERS

President Barack Obama doubled down on his campaign themes in a speech Tuesday, defending a robust government role for promoting economic growth and offering a pointed attack on Republican fiscal policy.

Obama has made the widening gulf between affluent Americans and average earners a dominant theme of his campaign, and he repeatedly returned to that point during a speech at Florida Atlantic University in Boca Raton. He advocated more government investment in research and education while denouncing the tax plans embraced by the Republican Party.

There's a debate going on in this country right now: Can we succeed as a nation where a shrinking number of people are doing really, really well but a growing number are struggling to get by, or are we better off when everybody gets a fair shot and everybody does a fair share and everybody plays by the same set of rules? Obama asked. This is not just another run-of-the-mill gabfest in Washington; this is the defining issue of our time.

Obama: Free Market Supported By Public Investment

The president tempered his statement that the free market is the greatest force for economic progress in human history by saying that society requires collective participation in building roads, instituting a system of public schools and equipping a military. He maintained that American prosperity has flowed from a strong middle class and that we cannot stop investing in the things that create that middle class.

This is not some socialist dream, Obama said in reference to investments in research and health care. They have been made by Democrats and Republicans for generations because they benefit all of us and they lead to strong and durable economic growth.

Rebukes Supply-Side Economics

Obama sideswiped his Republican presidential rivals in criticizing what he called discredited economic policies of slashing taxes and spending, saying that during the Bush administration, the rich got much richer, corporations made big profits, but we also had the slowest job growth in half a century.

A lot of folks who are peddling these trickle-down theories, including members of Congress and some people who are running for a certain office right now who shall not be named, they're doubling down on these old broken-down theories instead of moderating their views even slightly, Obama said.

Obama has gone on the offensive against a budget proposal authored by House Budget Committee Chairman Paul Ryan, R-Wis., and backed by likely Republican presidential candidate Mitt Romney, labeling the plan a radical vision and an experiment in Social Darwinism. He renewed those criticisms on Tuesday.

They proposed a budget that showers the wealthiest Americans with even more tax cuts, and they pay for these tax cuts by gutting investments in research, in clean energy, in health care, Obama said.

Obama also invoked Warren Buffett, the billionaire investor who has cited the fact that he pays a lower income tax rate than his secretary in pushing for higher taxes, and invoked a campaign touchstone -- the so-called Buffett Rule -- which would raise taxes on millionaires and billionaires.

The share of our national income going to our [top] one percent has climbed to levels we haven't seen since the 1920s, Obama said, clearly evoking the rhetoric of Occupy Wall Street. The folks benefiting from this are paying taxes at one of the lowest rates in 50 years.