Liberals descended on Pennsylvania Avenue Tuesday to protest President Barack Obama's decision to include entitlement cuts in his upcoming budget, delivering 2 million petitions demanding the White House back off its support for the chained CPI.

As we reported this weekend, liberals have been seething over the inclusion of the chained CPI in Obama's budget, which they see as a huge betrayal by the Democratic president.

This week, progressive groups, including MoveOn, the Progressive Change Campaign Committee, and Democracy for America, have mounted "emergency" online campaigns against the proposal, accusing Obama of turning on the very supporters who helped re-elect him to office.

Here's a sampling of the attacks, from MoveOn's Save Social Security tumblr:

MoveOn, PCCC, and Democracy for America are also threatening Democratic lawmakers against supporting entitlement cuts. Politico reports today that the three groups have sent "strongly worded letters" to Democrats in Congress warning that they could face a primary challenge from the left if they back Obama's proposals.

"It's really a slap in the face to Democrats who knock on doors and volunteer for campaigns," DFA spokesman Neil Sroka told Business Insider. "It hits at the very foundation of what it means to be a Democrat, and you're going to see primary challenges emerge."

"Any Democrat who votes to cut Social Security and Medicare is not a progressive Democrat, and they should be prepared to feel the ire of the progressive base."

But it is not clear if liberals have the resources or political will to back up these threats. For now, at least, there are a few reasons why the issue of entitlement reform is unlikely to cause a major split in the Democratic Party:

It doesn't look like Obama's budget is going to get anywhere. Both Republicans and Democrats have already criticized Obama's proposals, and a grand bargain seems highly unlikely.

Progressive groups are relatively weak. Compared to conservative outside groups like the Club for Growth and FreedomWorks, progressive organizations have fewer organizational and financial resources to mount serious primary challenges against incumbent lawmakers.

Obama's entitlement cuts are negligible compared to the sweeping entitlement reforms preferred by most Republicans.