Social Security checks at the US Treasury. (AP Photo/Bradley C. Bower)

US Senator Bernie Sanders, Congressman Mark Takano, Congressman Mark Pocan, Congressman Rick Nolan and leaders of organizations that oppose President Obama's anticipated assault on Social Security went to the White House Tuesday to present petitions signed by 2.3-million Americans who reject the president's proposal for “chained-CPI.”

The "chained-CPI" scheme would restructure cost-of-living adjustments in a way that cuts Social Security benefits for millions of seniors and veterans.

Sanders has vowed to “do everything in my power to block President Obama’s proposal to cut benefits for Social Security recipients through a chained consumer price index.”

And he's got allies. Joining the senator and the House members at the White House were representatives of Social Security Works, the National Committee to Preserve Social Security and Medicare, the National Organization for Women, the Progressive Change Campaign Committee, Democracy for America, the Campaign for America’s Future and MoveOn.org.

They were joined by Damon Silvers, the director of policy for the AFL-CIO, who announced that if the president goes forward with a budget that proposes Social Security cuts he will do so "without cover" from the labor movement.

The advocacy is significant, as Obama has yet to formally submit his budget. Senator Tom Harkin, D-Iowa, couples his outspoken opposition to the "chained-CPI" proposal with a message, "It's not too late to stop this."

It is the hope that Harkin is right that has inspired the dramatic response to news of the president's proposal. Americans really are, as Democracy for America chair Jim Dean notes, rising up in outspoken opposition to any cut in Social Security—but, especially, to a cut proposed by a Democratic president.

"Real Democrats don't cut social security benefits, period, and it's positively shameful that a Democratic President is leading the charge to do so," says Dean. "Protecting Social Security, Medicare and Medicaid benefits from cuts is a bedrock principle of the Democratic party. So, let's be clear: Any congressional Democrat who goes along with the plan the President is proposing and votes to cut Social Security benefits should be prepared to face the ire of the progressive base of the Democratic party and the primary challenges that come along with it."

Yet, most indications are that President Obama is still preparing a budget plan that would cut Social Security with the "chained-CPI" scheme. White House talking points regarding the budget plan still state that it includes "particular proposals in this plan like the CPI change."

The White House is already trying to soften the blow, with rhetorical flourishes and technical arguments, but Harkin is right when he says: "Call it whatever you want—the chained CPI is still a cut to those who need help the most."

By any reasonable measure, the fight over "chained-CPI" is, at this point, between progressives and the White House.

House Speaker John Boehner, R-Ohio, has already rejected the Obama budget plan—sight unseen. The question now is whether Obama will go ahead with the plan to compromise Social Security in order to reach a "grand bargain" that certainly appears to be out of reach.

If he does, it will be Obama who is putting Social Security cuts on the table—along with, some reports suggest, means testing for Medicare.

The president, who was reelected as the choice of Americans who seek to preserve Social Security, Medicare and Medicaid, may be prepared to abandon a commitment that has defined Democrats for decades.

Progressives have been blunt in stating that they will not follow Obama's lead if he proposes cuts. "Americans all over the country depend on every single dollar they get from Social Security to put food on the table and pay for housing," Congressional Progressive Caucus co-chairs Raul Grijalva, D-Arizona, and Keith Ellison, D-Minnesota, announced late last week. "Using chained CPI will shift more costs onto already struggling American families, seniors, veterans—including our 3.2 million disabled veterans who also depend on the Social Security calculation for their Veterans Affairs benefits—individuals with disabilities, and children on survivors’ benefits."

But the critical question is whether Democrats will tell a Democratic president that he cannot count on their support for cuts to Social Security.

Most House Democrats signed a February letter to the president that declares: "We remain deeply opposed to proposals to reduce Social Security benefits through use of the chained CPI to calculate cost-of-living adjustments. We remain committed to making the changes that will extend solvency for 75 years, but Social Security has not contributed to our current fiscal problems and it should not be on the bargaining table."

The 107 signers of that letter—which was circulated by Ellison, Grijalva, Jan Schakowsky of Illinois, John Conyers of Michigan and Donna Edwards of Maryland, with support from the likes of Minnesota's Nolan and Wisconsin's Pocan—make up a majority of the House Democratic Caucus. Another 30 House Democrats have signed or agreed to the sentiments of a letter (circulated by Florida Congressman Alan Grayson and Takano, a California Democrat) that says: "We will vote against any and every cut to Medicare, Medicaid or Social Security benefits—including raising the retirement age or cutting the cost of living adjustments that our constituents earned and need."

Those are the sentiments of FDR Democrats, who recognize that their party is—and should continue to be—defined by a historic commitment to maintain programs created by Franklin Delano Roosevelt, Harry Truman and Lyndon Johnson.

The problem, of course, is that pressures for loyalty to party principle are sometimes trumped by pressures for loyalty to the agenda of a party's sitting president.

Thus comes the test for congressional Democrats—and for their partisan constituents.

What will the Democratic party stand for in the budget debate?

President Obama still has it in his power to unite the party around the FDR standard.

If he fails to do so, however, congressional Democrats will have to decide whether there are some compromises that Democrats cannot accept.

All of us—especially President Obama—are Thatcherites now. Read Maria Margaronis's take.