Liberals balked Wednesday at recommendations from the leaders of President Obama's fiscal commission, specifically at proposals to curb Social Security benefits.

Sen. Bernie Sanders (I-Vt.), a leading liberal senator, and AFL-CIO President Richard Trumka quickly condemned new proposals from Democrat Erskine Bowles and Republican Alan Simpson, the co-chairmen of the president's commission, who released their initial recommendations for bringing down the long-term debt.

“The Simpson-Bowles deficit reduction plan is extremely disappointing and something that should be vigorously opposed by the American people," Sanders said in a statement.

Sanders has been among a group of congressional liberals who have threatened to defeat the commission's recommendations if it curtails Social Security benefits in any way. Sanders has said of the commission's recommendations that Congress would "vote it down" if it touched on Social Security, and Rep. Raúl Grijalva (D-Ariz.), joined by 136 other House Democrats, has written to similarly warn the commission.

The proposals released on Wednesday, charged Grijalva, co-chairman of the Congressional Progressive Caucus, would only favor the wealthy.

"The path this plan would set is not good for the public. Congress should be having a realistic, productive conversation right now about how to reduce our budget deficit and maintain a secure retirement system for those who have earned it," he said in a statement. "Instead, we’re debating a proposal from a commission dedicated to cutting crucial social programs and reducing corporate and upper-income taxes at the same time. This is not a recipe for a healthier American economy."

The new report suggests making Social Security more sustainable by steadily raising the retirement age over time, and adjusting the rate at which benefits increase every year. The commission chairmen propose other spending cuts and revenue reforms to balance the budget and bring down the debt. 

Trumka went after the commission's proposals in even stronger terms.

"The chairmen of the Deficit Commission just told working Americans to 'Drop dead,' " Trumka said in a statement. "Some people are saying this is plan is just a 'starting point.' Let me be clear, it is not."

The proposals put forward by Bowles and Simpson do not necessarily recommend the final product of the commission. Fourteen of the panel's 18 members from both parties must agree on final proposals in order to produce a final report, on which congressional leaders have pledged an up-or-down vote.

Sanders said the Social Security proposals were "reprehensible" and "impractical," and blamed Republicans for saying the entitlement program would go bankrupt.

“Despite all of the right-wing rhetoric, Social Security is not going bankrupt," he said. "According to the Congressional Budget Office, Social Security can pay every nickel owed to every eligible American for the next 29 years and after that about 80 percent of benefits."

Labor voices are represented on the commission, through Trumka is not a member. Former SEIU President Andy Stern is.

—This item was first posted at 2:56 p.m., and was updated at 3:14 p.m. and 3:33 p.m.