Vice President Joe Biden, who is weighing a run for the White House in 2016, gave a campaign-like speech to an international audience at a conference in Manhattan Thursday, declaring, "I have never been more optimistic about the possibilities of this great nation."

After being introduced by a former Senate colleague, Democrat George Mitchell, as one of the greatest legislators of his era -- of a rank with the late Sen. Edward Kennedy -- Biden urged investors, politicians and academics at the Concordia Summit on global affairs to combat corruption, fight for the middle class and believe in America's greatness.

Biden's deliberations over whether to run for president come as Democratic front-runner Hillary Rodham Clinton's poll ratings have wobbled. A USA Today/Suffolk University (Boston) poll released Thursday night showed Clinton supported by 41 percent of those likely to vote in Democratic primaries. Sen. Bernie Sanders (D-Vermont) got 23 percent, and Biden 20 percent.

Biden did not mention a candidacy in remarks at the Grand Hyatt New York that lasted nearly an hour. But he touted his decades in the Senate and the tasks given him by President Barack Obama to troubleshoot global strife and corruption, and manage relationships around the world, from Latin America to China to Russia and beyond.

"We have a chance to bend history just a little bit, if we just get out of our own way," Biden said. He added: "Folks, we got a lot to do."

He said he worries about the future of the middle class: "Without us figuring how to deal them back into the game, ladies and gentlemen, you're all in trouble. When the middle class does well, business does very well and the poor have a way up. What's America about? I can describe America to you in one word: possibilities."

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Biden also knocked the skepticism voiced by many conservatives that man-made pollution contributes to climate change.

"A lot of our friends on the other team, they don't think there's global warming," Biden said to applause. "They also deny gravity, um, and a few other things. It's just kind of fascinating."

Although Biden inveighed against the "concentration of wealth," he cautioned that he's "not a populist. . . . I'm a realist."

"I'm not Bernie Sanders," he said. "He's a great guy, by the way. No, he really is."