Sen. Bernie Sanders of Vermont, red in the face, took off his jacket and rolled up a shirt sleeve – but there was no relief from the discomfort of his affliction.

The poor guy is suffering from triangulation.

The man triangulating him, President Barack Obama, has proposed cuts to Social Security and Medicare as part of an attempt to find a middle ground in the budget debate. For Sanders, an independent and liberal member of the Senate Democratic caucus, the betrayal stung so badly that he literally took to the streets, joining left-wing activists for a protest Tuesday afternoon outside the White House.

“When Barack Obama was running for president in 2008, he said that he would not cut Social Security. We want the president to remember what he said and not go back on his word!” Sanders shouted into a microphone, as cops watched warily.

Calling Obama’s offer “nuts,” Sanders went on: “The White House tells us they want to defend the middle class – that’s their mantra. If you want to defend the middle class you don’t cut Social Security, you don’t cut Medicare, and you don’t cut benefits for disabled vets.”

“Say it!” somebody shouted.

“Give it to ’em!” somebody else called out.

Sanders’s ire was real, as was that of his fellow demonstrators: a pair of congressmen along with representatives of the AFL-CIO, the National Organization for Women, MoveOn.org, Campaign for America’s Future and the rest of the liberal establishment. “Inhumane,” they said of Obama’s proposal. “Dickensian … reprehensible.”

But, in reality, the progressives’ street protest did Obama a favor. He needs to have the likes of Bernie Sanders against him. It strengthens his hand and helps him negotiate a better deal with Republican leaders, who can now see that liberal backbenchers and interest groups can sometimes be as intransigent as conservatives.

At a Republican presidential debate in 2011, all eight candidates on the stage said they would reject a budget deal that raised taxes even if it had $10 of spending cuts for every dollar of tax increases. At Tuesday’s protest, I put the reverse question to participants: Could they accept a dollar of cuts in Medicare and Social Security benefits for every $10 of increased taxes on corporations and the wealthy? All those I asked said they would decline.

“Not for me, no,” said Rep. Rick Nolan, D-Minn.

“I’m not taking your offer,” said Stephanie Taylor, co-founder of the Progressive Change Campaign Committee.

“No, it’s not negotiable,” said Damon Silvers, the AFL-CIO’s policy director.

“Uh, no,” said Jim Dean, the chairman of Democracy for America.

Similar answers came from Roger Hickey of the Campaign for America’s Future, Manny Herrmann from MoveOn – and, of course, Sanders.

The liberals’ objections are legitimate – particularly their resistance to a stingier inflation formula for Social Security, which isn’t as big a budget problem as Medicare. There’s a case to be made that the president shouldn’t negotiate with himself by opening the bidding with his final offer. There’s also a concern that he now “owns” Social Security cuts, and Republicans can use that against him.

But Obama’s proposal, if the details turn out to be as advertised, restores his credibility on the budget. By skirting entitlement cuts, his previous budgets weren’t taken seriously.

Now Obama, by publicly defying liberals in his party, looks like the reasonable one – and Republicans look unreasonable if they continue to carp about Obama’s proposal without offering more tax hikes.

It’s perhaps the most brazen attempt at triangulation in the Democratic Party since Bill Clinton defied liberals on welfare reform. This worked well for Clinton, and it may work well for Obama – but in the short term he’s going to hear a lot of gasping and wheezing from those being triangulated.

Rep. Mark Takano, D-Calif., at the White House protest, complained that the proposed cuts would break “the sacred promises that we made to our nation’s seniors.”

Max Richtman of the National Committee to Preserve Social Security & Medicare called Obama’s justifications “malarkey.”

Nolan, the Minnesota congressman, was particularly agitated. He got in a showdown with police for standing too close to the White House; he dared the officers to arrest him.

Another speaker on the program, a Social Security beneficiary named Phyllis Zolotorow, got personal: “Believe me, Mr. President, this is not the way to honor your mother’s memory.”

The liberal activists cheered. They were angry with Obama – and defiant.

Jim Dean, Howard’s brother, shouted into the microphones: “The era of triangulation is over!”

Or is it just beginning?