Sen. Bernie Sanders (I-Vt.) on Friday aggressively pushed back on President Barack Obama's budget proposal, calling it a "bitter disappointment" and promising that the entitlement cuts contained in the plan wouldn't pass under his watch.

“I am terribly disappointed and will do everything in my power to block President Obama’s proposal to cut benefits for Social Security recipients through a chained consumer price index," Sanders said in a statement, blasting the plan to change how cost-of-living adjustments for Social Security payments are calculated.

As HuffPost explained on Friday, the proposal is highly unpopular among senior citizens:

Obama has proposed reducing future benefits by making annual cost-of-living adjustments less generous. Almost every year, the Social Security Administration adjusts benefits based on the prices of various consumer goods, including things like food and health care. Obama's proposal would change the way the government measures inflation from the current Consumer Price Index to what economists call "chained CPI."

Last year, beneficiaries received a 1.7 percent boost. If the adjustment had been calculated using chained CPI, seniors and other Social Security recipients would've received a 1.4 percent adjustment. The policy switch would save more than $100 billion over 10 years.

Sanders insisted Friday that the budget would "hurt a lot of people,” and noted earlier this week that it marked a reversal from Obama's 2008 position.

“In 2008, candidate Barack Obama told the American people that he would not cut Social Security," he said. "Having him go back on his word will only add to the rampant political cynicism that our country is experiencing today.”

As HuffPost reported on Friday, Sanders isn't the only one outraged at Obama over the proposal. Progressive-leaning groups quickly responded to the news by promising to mount primary challenges against congressional Democrats who sign on to the plan.

Also on HuffPost:

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  • Deficit Reduction

    The budget would reduce the deficit by $1.8 trillion over ten years -- $600 billion of this reduction would come from revenue raisers, and $1.2 trillion would come from spending reductions and entitlement reforms. <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>

  • Social Security

    It would change the benefit structure of Social Security (chained-CPI). <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>

  • Medicare

    It would means test additional programs in Medicare. <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>

  • Health Care Savings

    All told, it would include $400 billion in health care savings (or cuts). <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>

  • Additional Cuts

    It would cut $200 billion from other areas, <a href="http://www.nytimes.com/2013/04/05/us/social-programs-face-cutback-in-obama-budget.html" target="_blank">identified by The New York Times</a> as “farm subsidies, federal employee retirement programs, the Postal Services and the unemployment compensation system.” <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>

  • Access To Pre-K

    It would pay for expanded access to pre-K (an Obama priority) by increasing the tobacco tax. <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>

  • Retirement Accounts Limits

    It would set limits on tax-preferred retirement accounts for the wealthy, prohibiting individuals from putting more than $3 million in IRAs and other tax-preferred retirement accounts. <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>

  • Overlapping Benefits

    It would stop people from collecting full disability benefits and unemployment benefits that cover the same period of time. <a href="http://www.huffingtonpost.com/2013/04/05/obama-budget_n_3019281.html?utm_hp_ref=politics" target="_blank">(read more)</a>