Obama told a throng of some 800,000 people on Washington's National Mall - an aerial estimate tallied the crowd at up to 1 million - that "preserving our individual freedoms ultimately requires collective action. For the American people can no more meet the demands of today's world by acting alone than American soldiers could have met the forces of fascism or communism with muskets and militias."
He spoke on the national Martin Luther King holiday and took the oath on a Bible belonging to the slain civil-rights hero - 150 years after the Emancipation Proclamation and 50 years after King's Birmingham campaign. And the first black president placed his progressive ideals amid a struggle for full equality for women, African-Americans, gays and other minorities that "guided our forebears through Seneca Falls, and Selma, and Stonewall."
Here in Philadelphia, that last reference brought tears to the eyes of Mark Segal, the Philadelphia Gay News publisher, who took part in those Stonewell riots for gay rights in New York's Greenwich Village in the summer of 1969, when he was a teenager.
"You've never seen a president of the United States, during his inauguration, talk about full equality for gays and lesbians, and to equate Selma [the 1965 march for black voting rights] with Stonewall - it gave me chills," he said.
Obama's speech drew not only that emotional reaction but also praise from the likes of former Jimmy Carter speechwriter and liberal writer James Fallows - who said the president "confounded expectations" with "the most sustainedly 'progressive' statement Barack Obama has made in his decade on the national stage."
It was proof that four years of GOP filibusters, obstructionism and debt-ceiling-hostage crises have persuaded Obama to pursue a very different style of leadership - one in which mushy compromise is no longer his opening gambit. Instead, he will dance with the ones "who brung him" to a second term - a majority coalition of Latinos, blacks, liberal college-educated whites, gays, and Asians, among others, that wants its agenda pursued.
"It's pretty clear from the way that he handled the debt-ceiling fight that his second term is going to be a lot different from his first," said James Peterson, a Lehigh University professor who specializes in youth culture. He said he believes that Obama is looking to reward the voters who put him back into office in November - many of whom don't think his first term was liberal enough.
The boldness of Obama's speech was driven home by his words on income inequality and the plight of America's poor - arguably the boldest since Lyndon Johnson declared a "War on Poverty" in 1964. "We do not believe that in this country, freedom is reserved for the lucky, or happiness for the few," Obama declared.
Needless to say, Republicans are not thrilled with Obama's newly assertive liberalism. California GOP Rep. Darrell Issa told Politico after the speech, "I'm hoping that the president will recognize that compromise should have been the words for today, and they clearly weren't."
Not all liberals were in a state of unrestrained ecstasy, either. As Obama spoke, about 10 protesters played dead in sheets splattered with blood-red paint to denounce the White House's secretive drone missile strikes that have killed both terrorists and civilians from Pakistan to Yemen, an issue the president selectively excluded from Monday's address.
And now comes the hard part: getting that progressive agenda past a Congress in which Republicans still control the House and have a filibuster-size minority in the Senate. As the Obama-loathing website the Drudge Report was quick to point out on Monday, he has just 1,461 days left to get that done.
On Twitter: @Will_Bunch