Today may mark a nadir in the often strained relations between President Obama and the "professional left," as Robert Gibbs derisively called the progressive movement, which has been pushing for months to prevent Obama from endorsing cuts to the social safety net. When the White House released its budget today, it was clear the president hadn't listened to the increasingly urgent threats and pleadings from the people who helped elect him, as his spending proposal includes a change to the way Social Security benefits are calculated, called the chained CPI, as well as changes to Medicare.

The cut is anathema to liberal activists and lawmakers, who have gone all in to oppose the cut, warning Obama would face a "huge backlash" from his own base if he endorsed it. Now they're threatening primary challenges against any Democrats who vote for their president's budget.

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"You can't call yourself a Democrat and support Social Security benefit cuts," said Stephanie Taylor, co-founder of the Progressive Change Campaign Committee, a group which endorses liberals in primaries, sometimes against establishment picks. "The president is proposing to steal thousands of dollars from grandparents and veterans by cutting cost of living adjustments, and any congressional Democrat who votes for such a plan should be ready for a primary challenge. Social Security is the core of the progressive and Democratic legacy. The President has no mandate to cut these benefits, and progressives will do everything possible to stop him."

Over 100 Democratic House members -- more than half of the caucus -- have already signed a letter pledging to vote against any bill that includes entitlement cuts, meaning Obama may well have to rely on Republicans to pass his budget.

"Millions of MoveOn members did not work night and day to put President Obama into office so that he could propose policies that would hurt some of our most vulnerable people," said MoveOn.org Executive Director Anna Galland. "Just as we fought and defeated President Bush's plan to privatize Social Security, we will mobilize and stop this attempt to diminish the vital guarantee of Social Security... Every member of Congress -- Democrat or Republican -- who votes for this proposal should expect to be held accountable."

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Most of all, the left feels betrayed. Alex Lawson, the executive director of the advocacy group Social Security Works, noted that Obama said in his 2011 State of the Union Address that he wouldn't cut Social Security. "Today we learned that his budget breaks that promise and risks the legacy of Democrats going back to FDR by offering to immediately cut the earned benefits of those least able to afford it," he said.

Last month, Sen. Elizabeth Warren, a progressive hero, told me, "'Chained CPI’ is just a fancy way to say ‘cut benefits for seniors, the permanently disabled, and orphans.' Our Social Security system is critical to protecting middle-class families, and we cannot allow it to be dismantled inch by inch.”

While chained CPI has been called by some the least bad way to cut the social safety net, even liberals who support it often say it should only be used as a major concession to Republicans to get something Democrats want. But by putting it on the table now, Obama has already pre-conceded to demands that no Republicans have specifically made, and finds himself in the awkward position of being the only major player in either party to suggest cutting Social Security.

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UPDATE: Democracy for America communications director Neil Sroka told Salon that any Democrat who votes to cut Social Security, Medicare, or Medicaid benefits would run afoul of his group as well and should expect a primary challenge. "Plain and simple, siding with the president in support of these cuts is tantamount to declaring war on the Democratic base, which helped elect each and every member of the Democratic Party," he said.