President Obama delivered a stern and stinging rebuke of the Republican vision for the country Tuesday, castigating the GOP as a “radical” party that has strayed so far from the political middle that its policies represent an affront to core American values.

Singling out GOP presidential front-runner Mitt Romney for the first time, Obama sought to lump all Republicans under an ideological umbrella that, he argued, has shifted far from the days of the party’s more moderate icons such as Abraham Lincoln and Ronald Reagan. Reagan, he said, would be unelectable in the modern Republican Party.

Obama said the House Republican budget plan, which has been endorsed by Romney and would slash entitlements and agency spending, is “so far to the right” on the political spectrum that it makes the Republicans’ 1994 Contract With America “look like the New Deal.”

“This isn’t a budget supported by some small rump group in the Republican Party,” Obama said. “This is now the party’s governing platform. This is what they’re running on.”

Mocking Romney for calling the House budget “marvelous,” the president added that the plan, which aims to trim $5.3 trillion from federal spending over the next decade, would create a form of “social Darwinism” pitting the poor against the wealthy.

“It’s a Trojan horse,” he declared in an address to news editors in Washington. “Disguised as a deficit-reduction plan, it’s really an attempt to impose a radical vision on our country. . . . It’s a prescription for decline.”

The speech was the third and by far the most aggressively partisan that Obama has given since late last year on what he describes as the country’s drift from its principles — most particularly a sense of economic fairness — that he contends has damaged the middle class. He did not attack Republicans on Tuesday simply on their policies but also on their idea of what it means to be an American, calling their vision “antithetical to our entire history.”

The president’s blunt demeanor opened a new front in the 2012 campaign in a week when Romney moved closer to securing the Republican nomination. Obama’s tone marked a sharp departure from his more nuanced approach of last summer, when he distinguished between the moderate and conservative wings of the GOP while trying to win bipartisan agreement on a “grand bargain” to reduce the budget deficit.

By veering from his 2008 campaign pledge to instill a spirit of bipartisanship in Washington, Obama is gambling that he can convince voters, especially the decisive bloc of independents, that his opponents are the ones to blame.

His attack also marked the second time in two days that the president attempted to paint conservative leaders in Washington as out of step with the majority of the public. On Monday, he warned the conservative wing of the Supreme Court to refrain from judicial activism in determining the fate of his health-care law.

Paul Ryan hits back

The author of the House budget, Rep. Paul Ryan (R-Wis.), quickly blasted Obama for choosing to “duck and run” instead of dealing with the burgeoning national debt, which Republicans have cited as a drag on economic growth and consumer confidence.

“Like his reckless budgets, today’s speech by President Obama is as revealing as it is disappointing: While others lead by offering real solutions, he has chosen to distort the truth and divide Americans in order to distract from his failed record,” Ryan said in a statement.

Appearing on conservative talk-radio host Sean Hannity’s show, Romney charged that Obama had misrepresented his policies and those of the Republican Party. Furthermore, he said, the president “fails to acknowledge the mistakes and the errors in his own record. It’s just astonishing to listen to him.”

White House aides billed the speech before the American Society of News Editors as the third in a series of major addresses Obama has used to lay out a populist agenda that calls for government to play an active role in helping build a more equitable society. Ina speech at a high school in Osawatomie, Kan., in December, he invoked the progressive populism of Theodore Roosevelt, and in his State of the Union address before Congress in January, he called for an “economy built to last.”

By choosing to give Tuesday’s speech to a room full of newspaper editors, the president sought to ensure that his argument will remain vivid in the minds of those overseeing campaign coverage in the crucial months ahead.

Though the House budget plan faces likely defeat in the Democrat-controlled Senate, Republicans have sought to use the legislation as a marker to frame the parties’ election-year debate on fiscal issues. On Tuesday, Obama aimed to further accentuate the differences between the parties, using the Ryan plan as a metaphor for a GOP vision that he said counters the nation’s traditional promise of a path for upward mobility for the middle class.

Blaming Bush

Obama blamed the growing deficit primarily on what happened under his predecessor, George W. Bush: “two wars, two massive tax cuts and an unprecedented financial crisis.” And he faulted Republican policies, including tax cuts for the wealthy and lax financial regulation, for leading the country into the Great Recession in 2008.

“You would think that after the results of this experiment in trickle-down economics, after the results were made painfully clear, that the proponents of this theory might show some humility, might moderate their views a bit,” Obama said. “But that’s exactly the opposite of what they’ve done. Instead of moderating their views even slightly, the Republicans running Congress right now have doubled down.”

Listing the implications if the Ryan budget were implemented — 10 million college students with higher loan payments, 200,000 children denied early-education programs, 4,500 fewer federal grants to fight crime — Obama cast the election as a fundamental choice for the public on what kind of future the country should have.

In tough times, he said, “the debate gets sharper; it gets more vigorous. That’s a good thing.”

Wrapping up, Obama emphasized that his call for a shared sense of responsibility “isn’t a partisan feeling. This isn’t a Democratic or Republican idea. It’s patriotism.”