The ambitious “Great Society” agenda begun half a century ago continues to touch nearly every aspect of American life. But the deep philosophical divide it created has come to define the nation’s harsh politics, especially in the Obama era.

On the 50th anniversary of President Lyndon B. Johnson’s declaration of a War on Poverty, Republicans and Democrats are engaged in a battle over whether its 40 government programs have succeeded in lifting people from privation or worsened the situation by trapping the poor in dependency.

Many of today’s fiercest political debates can be traced to the aspirations of the Great Society, the domestic programs it spawned during the 1960s, and the doubts it raised about the role and reach of Washington.

Johnson’s years in office saw the greatest expansion of government since Franklin D. Roosevelt’s New Deal, and even exceeded the scope of those Depression-era programs. The two parties have been fighting about it ever since.

“It is a kind of uniquely American thing that we have had all these battles over the size of government,” said Neera Tanden, president of the Center for American Progress, a liberal think tank. “The attack on government has been with us for decades, and it has been very effective. Progressives are able to overcome it when the problems loom largest.”

This may prove to be such a moment.

With battles underway over extending long-term unemployment benefits and raising the minimum wage, leaders of both parties are feeling a new urgency to address the issue of rising income inequality, as well as the economic insecurity that many in the middle class are feeling. But to a large degree, that means coming to terms with a political legacy that reaches back to the 1960s.

“We created new avenues of opportunity through jobs and education, expanded access to health care for seniors, the poor, and Americans with disabilities, and helped working families make ends meet,” President Obama said in a statement commemorating the War on Poverty’s 50-year mark. “But as every American knows, our work is far from over.”

The White House’s Council of Economic Advisers released statistics showing that the percent of the population in poverty, measured to include tax credits and other benefits, has declined by a third since 1967, from 25.8 percent to 16 percent.

And in one clear echo of the Great Society, Obama announced Tuesday that five American cities have been designated “promise zones,” where the government will provide tax incentives and grants to address poverty.

But even as Obama celebrated the War on Poverty, GOP leaders were citing it as proof of liberalism’s failure.

In a speech at the conservative American Enterprise Institute, Sen. Marco Rubio (Fla.), a potential 2016 presidential contender, noted, “Five decades and trillions of dollars after President Johnson waged his War on Poverty, the results of this big-government approach are in.

Some statistics about the Supplemental Nutrition Assistance Program.

“We have 4 million Americans who have been out of work for six months or more. We have a staggering 49 million Americans living below the poverty line, and over twice that number — over 100 million people — who get some form of food aid from the federal government,” Rubio said. “Meanwhile, our labor force participation is at a 35-year low, and children raised in the bottom 20 percent of the national income scale have a 42 percent chance of being stuck there for life.”

Johnson announced the War on Poverty in his 1964 State of the Union address. Four months later, in a commencement speech at the University of Michigan, he put forward a more far-reaching vision, declaring that “we have the opportunity to move not only toward the rich society and the powerful society, but upward to the Great Society.”

It would ultimately include a raft of initiatives: Medicare and Medicaid, the first direct federal aid to school districts, Head Start, food stamps, landmark environmental legislation, the Job Corps to provide vocational education, urban renewal programs, national endowments for the arts and humanities, civil rights legislation, funding for bilingual education.

The political environment was ripe: The nation was reeling from the assassination of a young, idealistic president; Johnson had a firm hold on his governing coalition; and the economy was booming.

“Nineteen sixty-four was the beginning of about 11 or 12 years of massive change in the role of government, that went through Johnson and the Nixon years,” said Charles Murray, a political scientist and scholar at the American Enterprise Institute, whose influential 1984 book “Losing Ground” argued that the Great Society made the problems of the poor and disadvantaged worse.

But even as Johnson was cruising to a landslide reelection victory, the terms of engagement were being set for a longer battle. Conservatives often say it began when a Hollywood actor, who was co-chairman of Californians for Barry Goldwater, made a nationally televised last-ditch appeal for the GOP presidential nominee.

“In this vote-harvesting time, they use terms like the ‘Great Society,’ or as we were told a few days ago by the president, we must accept a greater government activity in the affairs of the people,” Ronald Reagan said. “A government can’t control the economy without controlling people. And they know when a government sets out to do that, it must use force and coercion to achieve its purpose.”

Goldwater’s campaign was astonished when its switchboard lit up with calls pledging money, Reagan later wrote. “The speech raised $8 million and soon changed my entire life.”

In a struggle that lasted through much of the 1970s, the conservative movement took over the Republican Party. Reagan was elected president in 1980 — in part, on his promise to dismantle much of the Great Society.

The programs, by and large, endured.

“Reagan lost, and the Great Society won,” said William Galston, a senior fellow at the Brookings Institution who was a top policy adviser to President Bill Clinton. “If you look at the big things that happened during that period, most of them are still with us, and costing more than ever.”

But resentment and disillusionment with many poverty programs grew. In the 1990s, Clinton achieved something of a cease-fire when he declared that “the era of big government is over,” and overhauled the welfare system.

“Clinton very consciously set out to thread the needle on the portions of the Great Society that had become most unpopular with middle America,” Galston said.

But there remained between the two parties philosophical disagreements about the role of government, and resulting fiscal ones over how much of the country’s resources should go toward paying for it.

Obama’s election came at a time of deep economic crisis, as well as the opportunity presented by the fact that Democrats had control of the executive and legislative branches.

For liberals, that meant a chance to pass broad health-care legislation, which many considered unfinished business after Medicare and Medicaid. But that also brought a political backlash, which cost Democrats the House in 2010.

Some think a new dynamic is taking shape, as the economic recovery fails to lift those on the lowest rungs and to reassure the middle class.

“It seems entirely possible to me that by the 2016 election, the public will be ready to entertain a much bolder set of ideas about creating a more shared prosperity,” said Robert Reich, who was labor secretary under Clinton.

And even conservatives say they cannot risk appearing insensitive to the anxieties and insecurities of those who are struggling.

“The left doesn’t work, and the right has been indifferent,” said Newt Gingrich (Ga.), a former House speaker who remains one of the nation’s most prominent conservatives. “What you need is a new reform centrism that says, ‘We’re going to take the values of the right and the concerns of the left.’ ”