President Obama speaks about the economy, Tuesday, Dec. 6, 2011, at Osawatomie High School in Osawatomie, Kansas. (Carolyn Kaster/AP)

President Obama, in one of his most expansive speeches to date, declared on Tuesday that supply-side economics is a failure and called “gaping inequality” across the country a moral shortcoming that is distorting American democracy.

Obama’s speech in Kansas was not just another attack on Congress, or a plea to pass his jobs bill. He did not roll out a new, snappy slogan – such as telling the audience that “we can’t wait” to enact new laws.

Instead, Obama delivered a searing indictment of Republican economic theory, setting the stage for the coming presidential campaign. Summoning the image of a populist Theodore Roosevelt — in the same town (Osawatomie) where Roosevelt delivered a famous speech on economic fairness in 1910 — Obama deployed the language of right and wrong, fairness and unfairness, in a lengthy address that aides said he largely wrote himself.

The theory of “trickle down economics,” which holds that greater wealth at the top generates jobs and income for the masses below, drew some of Obama’s harshest criticism.

“It’s a simple theory — one that speaks to our rugged individualism and healthy skepticism of too much government. It fits well on a bumper sticker. Here’s the problem: It doesn’t work,” Obama said of supply-side economics, drawing extended applause. “It’s never worked.”

He repeatedly conjured the image of a country that is becoming more divided by inequality. He linked the Tea Party to the Occupy Wall Street protesters, saying it is little wonder that the “breathtaking greed of a few” who caused the financial crisis has generated a “raging debate over the best way to restore growth and prosperity, balance and fairness.”

“This isn’t just another political debate. This is the defining issue of our time,” he said. “This is a make or break moment for the middle class, and all those who are fighting to get into the middle class.

“At stake is whether this will be a country where working people can earn enough to raise a family, build a modest savings, own a home, and secure their retirement.”

Although the unemployment rate has been a constant shadow hanging over Obama’s presidency, the mechanics of job growth had only a small part in the speech, which dwelled as much on the need for infrastructure investments, better education and a tax code that Obama said “must reflect our values.”

And after many weeks of rolling out bite-sized initiatives, Obama’s grander ambitions were on full display.

He said it is “heartbreaking” that millions of working families are forced to take their children to food banks. “But the idea that those children might not have a chance to climb out of that situation and back into the middle class, no matter how hard they work?” he said. “That’s inexcusable. It’s wrong.”

The line drew some of the most rousing applause.

With so much attention devoted to the Republican primary contest in recent days, the Obama speech offered an opportunity for him to lay down a marker against any of the potential Republican nominees, especially Mitt Romney or Newt Gingrich.

For Gingrich, perhaps, there were special digs: Obama made two references to the child labor laws that were enacted at the beginning of the past century. The former House speaker has recently suggested that poor children could be put to work in certain circumstances, such as mopping the floors at schools.

In the early 20th century, Obama said, the country faced a “choice” whether to allow “even our children to work ungodly hours in conditions that were unsafe and unsanitary.” Roosevelt, Obama said a few moments later, fought to ensure companies could not profit “by exploiting children.”

Obama also seemed to stake out a more modest position on the current debate in Congress over whether to extend a payroll tax cut. Although the president had proposed expanding the tax cut even further, so that both employers and employees would keep more of their money, on Tuesday he called simply to “extend a payroll tax cut,” suggesting he could settle for the tax cut as it is currently configured.