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San Antonio Mayor Julian Castro gave a ringing endorsement of President Obama on Tuesday in a convention keynote address that also blasted Mitt Romney for embracing policies that would "pummel" the middle class.

Castro, the youngest mayor of a major U.S. city, also made history as the first Hispanic to deliver a keynote address at the Democratic National Convention -- a role that propelled the 37-year-old from political obscurity to national prominence.

Castro criticized Romney and his running mate, Rep. Paul Ryan of Wisconsin, for championing a budget plan he said "dismantles" the middle class.

"We know that in our free market economy some will prosper more than others," Castro said. "What we don't accept is the idea that some folks won't even get a chance. And the thing is, Mitt Romney and the Republican Party are perfectly comfortable with that America. In fact, that's exactly what they're promising us."

Castro also sought to portray Romney as anti-women's rights, and hit the former Massachusetts governor for what he described as a reversal in his view on health care.

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    "When it comes to respecting women's rights, Mitt Romney says, 'No.' When it comes to letting people marry whomever they love, Mitt Romney says, 'No.' When it comes to expanding access to good health care, Mitt Romney … actually ... Mitt Romney said, 'Yes,' and now he says, 'No.'"

    The last remark drew a roar from the crowd, acknowledging his reference to the Massachusetts health care law Romney passed as governor. That law was said to have been a model for President Obama's federal health care overhaul, which has been a top target of Republicans.

    "Governor Romney has undergone an extreme makeover, and it ain't pretty," Castro said. "So here's what we're going to say to Mitt Romney. We're going to say, 'No.'"

    The attacks came after Republicans used their convention last week to make the case that Romney is the best choice to help the middle class, while slamming Obama for what they said was a failure to deliver on his promises.

    "President Obama promised to begin to slow the rise of the oceans and to heal the planet," Romney said in his acceptance speech. "My promise is to help you and your family."

    Castro was introduced Tuesday night by his closest adviser: his identical twin brother, Joaquin, a Texas state legislator who is favored to win election to Congress this year.

    In his highly coveted keynote address, Julian Castro introduced himself as the embodiment of the American dream.

    "My family's story isn't special," said Castro. "What's special is the America that makes our story possible. Ours is a nation like no other, a place where great journeys can be made in a single generation. No matter who you are or where you come from, the path is always forward."

    Castro's family narrative begins with his grandmother, a Mexican orphan, who immigrated as a young girl to the U.S. in 1920 and taught herself how to read and write. Castro and his brother were raised by their single mother, Rosie, whom the two credit for leading them toward a career in public service.

    "My grandmother spent her whole life working as a maid, a cook and a babysitter, barely scraping by, but still working hard to give my mother, her only child, a chance in life, so that my mother could give my brother and me an even better one," he said. "My mother fought hard for civil rights so that instead of a mop, I could hold this microphone."

    Castro went on to tout Obama's record as president, while also promote his own causes as mayor of San Antonio -- in particular, education reform.

    "When Detroit was in trouble, President Obama saved the auto industry and saved a million jobs," Castro said.

    "Seven presidents before him — Republicans and Democrats — tried to expand health care to all Americans. President Obama got it done," he said. "He made a historic investment to lift our nation's public schools and expanded Pell grants so that more young people can afford college. And because he knows that we don't have an ounce of talent to waste, the president took action to lift the shadow of deportation from a generation of young, law-abiding immigrants called dreamers."

    The keynote speaking slot propelled Castro into the national spotlight Tuesday, further fueling speculation that the young mayor -- who few had heard of before he was selected for the address -- is eyeing higher office, possibly as governor of Texas or even the country's first Hispanic president. It was just eight years ago when then-Senator Obama used the same primetime speaking slot to rise to national prominence.

    The decision to tap Castro as the convention's keynote speaker also speaks to the Democratic Party's push to secure Hispanic voters -- the country's fastest growing minority and a critical voting bloc in the election.