Bernie Sanders offered a polemical speech at a socialist conference in Mexico City on Monday that showcased his populist economic message, but was marked by accusations from his opponents that he is in fact a Democrat.

“I may be a Democratic socialist, but to call me a socialist is a lie,” the Vermont senator declared.

A representative for Mr. Sanders told the audience at the one-day conference, hosted by the International Marxist Tendency, that it could make Mr. Sanders’ proposed cuts to Social Security and Medicare available to other states, such as California.

Several people in the audience erupted into applause.

The senator was preceded on stage by Sen. Tammy Baldwin of Wisconsin, who accused Republicans of “extreme ideas” like the merit pay for teachers and privatizing schools.

Mr. Sanders introduced Howard Zinn, an American historian and social-democratic activist who promoted the progressive platform that Mr. Sanders proposes on the campaign trail last year.

He also addressed Bernie 2016 bumper stickers, which Mr. Sanders claimed were, contrary to popular belief, merely a reference to the 2020 campaign.

“They are not ‘Feel the Bern’ stickers,” he said. “They are Bernie stickers.”