President Obama’s budget is due early next month, and already progressives inside and outside Congress are pushing back against one potential line in the document — the Social Security cuts, in the form of “chained CPI,” that Obama has included in his budget several times in the past.

Late last week, 16 senators ranging from liberal independent Bernie Sanders to Democrat Mark Begich, who is facing a tough reelection in the red state of Alaska this fall, signed a letter to the White House asking the president not to include the cuts in his budget.

Meanwhile, progressive activists are getting aggressive on the issue. Campaign for America’s Future and Social Security Works have teamed up to urge progressives to tell the White House that chained CPI would be unacceptable.

Chained CPI is a change in how the government calculates inflation, which among other things would result in a slower rate of growth in Social Security benefits. Some economists say that chained CPI is a “more accurate” measure of inflation, but opponents of the policy persuasively argue that the elderly face a different sort of inflation, driven largely by spending on out-of-pocket health care, and that chained CPI cuts benefits when they should be expanded.

Obama’s chained CPI proposal from last year would take $9,521 in cumulative benefits from an average 85-year-old on Social Security, even with protections included in the proposal — including a small benefit bump at age 75 and protections for the very poor retirees.

Already this year, progressives have had to stomach $8.6 billion in food-stamp cuts in the farm bill Obama signed, along with Congress’s failure to extend unemployment benefits for the long-term jobless.

Amid the economic downturn, especially for lower- and middle-income Americans, many liberals say they can’t accept these cuts to Social Security, too. “These are tough times for our country. With the middle class struggling and more people living in poverty than ever before, we urge you not to propose cuts in your budget to Social Security, Medicare and Medicaid benefits – cuts which would make life even more difficult for some of the most vulnerable people in America,” the letter from Senate Democrats said.

There is a strategic element here as well. White House officials have explicitly argued in the past that chained CPI isn’t their preferred policy but a concession Obama would be willing to make in exchange for increased tax revenue and new spending on such things as education and infrastructure.

But many progressives, of course, see it differently. In an election year, and one in which immigration reform will be the main issue anyhow, the chances that Congress passes some sort of grand bargain on entitlements and new spending are somewhere around zero.

In that context, many progressives think Social Security cuts on the table is a dangerous move. They think this is true in the short term, because as you may recall, the head of the National Republican Congressional Committee attacked Democrats last year when chained CPI was in Obama’s budget.

They also think it’s unwise in the long term, because by repeatedly making a pre-concession of Social Security cuts year after year, Democrats mainstream the idea and make eventual benefit reductions inevitable. Liberals have been making this argument for years, and the fact that chained CPI keeps showing up in Obama’s budget seems to prove their point.