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With union members chanting “Bernie! Bernie!” behind him, Senator Bernie Sanders sought to burnish his populist credentials for president on Thursday as he introduced a bill to protect the pensions of millions of retirees from the threat of deep cuts.

Mr. Sanders railed against a spending bill approved by Congress last December — and signed into law by President Obama — that included a provision allowing certain multiemployer pension plans to make deep cuts in benefits to retirees to shore up their finances.

At a news conference in Washington, Mr. Sanders said the economic system was “rigged” against working-class retirees. With wealthy Americans given tax breaks and myriad financial perks, “the very least that we can do is to keep our promises to people who worked and earned their pensions,” he said.

“We say loudly and clearly that’s not what America is about!” he shouted, jabbing his hand in the air. “Enough is enough!”

Members of the Teamsters union and the International Association of Machinists and Aerospace Workers cheered him on as he spoke, waving banners that read, “Keep Pension Promise.”

Mr. Sanders, a socialist from Vermont who is running for the Democratic nomination, has sought to outflank Hillary Rodham Clinton on the left, attacking her positions on contentious issues like trade and the Keystone oil pipeline.

Mr. Sanders said the provision approved by Congress could mean cuts of up to 30 percent in the pensions of 10 million people. A bill introduced in the Senate by Mr. Sanders and in the House by two Ohio Democrats, Marcy Kaptur and Tim Ryan, would create a guaranteed federal fund to prevent that from happening.

When asked how his populist message was resonating with voters, Mr. Sanders waved off the question, saying, “I don’t want to make this into a political issue.”

He then began another rousing defense of employee rights, drawing more applause from the union members standing behind him.