Senator Elizabeth Warren (D-Mass.) made it clear Wednesday in an email to supporters that not only would she oppose President Barack Obama's plan to cut Social Security benefits through a cost-of-living adjustment known as chained CPI, but that she was "shocked to hear" it was included in the White House's budget proposal at all.

Warren said her brother David lives on the $13,200 per year he receives in Social Security benefits. "I can almost guarantee that you know someone -- a family member, friend, or neighbor -- who counts on Social Security checks to get by," she wrote. She continued:

That's why I was shocked to hear that the President's newest budget proposal would cut $100 billion in Social Security benefits. Our Social Security system is critical to protecting middle class families, and we cannot allow it to be dismantled inch by inch.

The President's policy proposal, known as "chained CPI," would re-calculate the cost of living for Social Security beneficiaries. That new number won't keep up with inflation on things like food and health care -- the basics that we need to live.

In short, "chained CPI" is just a fancy way to say "cut benefits for seniors, the permanently disabled, and orphans."

Two-thirds of seniors rely on Social Security for most of their income; one-third rely on it for at least 90% of their income. These people aren't stashing their Social Security checks in the Cayman Islands and buying vacation homes in Aruba – they are hanging on by their fingernails to their place in the middle class.

While the inclusion of the proposal should perhaps not be "shocking" -- it's long been on offer on the White House's website -- adopting the inflation measure would have an immediate impact on vulnerable Americans. As Walter Hickey reported, "a switch to chained CPI from the current rate demonstrably cuts benefits to seniors and could be a stealth tax on primarily the middle class." Citing research conducted by the Center for Economic and Policy Res..., Hickey goes on to note:

Switching to the Chained CPI immediately would have a more significant impact on the retirement income of seniors than the ending the Bush-era tax cuts would have on the after-tax income of the wealthiest 2 percent of households. For the average worker retiring at age 65, this would mean a cut of about $650 each year by age 75 and a cut of roughly $1,130 each year at age 85.

Hence, Warren's concern for people like her brother. But her opposition to chained CPI is also rooted in a simple campaign promise she made during a debate against then-Sen. Scott Brown: "I want to make clear: I will not go to Washington to cut Medicare or Social Security benefits."

There is widespread distate among Democrats for chained CPI, but as HuffPost's Arthur Delaney reported back in December, that doesn't always translate into the sort of opposition that Warren and her Senate colleague Bernie Sanders (I-Vt.) are willing to offer. Significantly, House Minority Leader Nancy Pelosi (D-Calif.) does not consider chained CPI to be a benefit cut.

As Alex Pareene noted, however, the best hope for seniors, at least for the time being, is that "intransigent House Republicans" are likely to blow up the "Grand Bargain."

The complete text of Warren's email is as follows:

Hi everyone,

My brother David has always had the special spark in our family.

Like our two older brothers, David served in the military. When he got out, he started a small business -- and when that one didn't work out, he started another one. He couldn't imagine an America where he wasn't living by his wits every single day.

Year after year, my brother paid into Social Security. He never questioned it. He figured he was paying so that he -- and a lot of other people -- could have a secure retirement.

Today my brother lives on his Social Security. That's about $1,100 a month. $13,200 a year.

I'm telling you my brother's story not because it's unusual, but because it's like the story of so many other people. I can almost guarantee that you know someone -- a family member, friend, or neighbor -- who counts on Social Security checks to get by.

That's why I was shocked to hear that the President's newest budget proposal would cut $100 billion in Social Security benefits. Our Social Security system is critical to protecting middle class families, and we cannot allow it to be dismantled inch by inch.

The President's policy proposal, known as "chained CPI," would re-calculate the cost of living for Social Security beneficiaries. That new number won't keep up with inflation on things like food and health care -- the basics that we need to live.

In short, "chained CPI" is just a fancy way to say "cut benefits for seniors, the permanently disabled, and orphans."

Two-thirds of seniors rely on Social Security for most of their income; one-third rely on it for at least 90% of their income. These people aren't stashing their Social Security checks in the Cayman Islands and buying vacation homes in Aruba – they are hanging on by their fingernails to their place in the middle class.

My brothers and I grew up in an America that invested in its kids and built a strong middle class. An America that allowed millions of children to rise from poverty and establish secure lives. An America that created Social Security and Medicare so that seniors could live with dignity.

We can't chip away at America's middle class and break the promise we make to our seniors.

Thank you for being a part of this,

Elizabeth

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CORRECTION: This story originally stated Warren's brother received $13,200 per month in Social Security benefits. It has been updated to reflect he actually receives $13,200 per year.