A Democratic National Convention vital to President Barack Obama's re-election bid opened Tuesday with praise for his battles on behalf of minority rights and the middle class, as well as plentiful attacks on Republican challenger Mitt Romney.

Facing a tight race in November and Republican attacks that they have made things worse while in power, Democratic organizers want their three-day convention to emphasize the tough decisions Obama has made and the additional steps needed to bolster the middle class.

The more than 5,500 Democratic delegates adopted a party platform that emphasizes middle-class opportunity while differing sharply on key issues with the Republican version endorsed at last week's GOP convention.

Then speech after speech lambasted Romney and Republicans, accusing them of being out of touch and politically divisive at a time requiring national unity to confront high unemployment, a sluggish economy and mounting federal deficits and debt.

Seeking to further strengthen Obama's advantage with women, Hispanic Americans and young voters, the Democratic speakers hailed the president for promoting health care reforms, supporting gay marriage, and ending deportations of some young illegal immigrants.

First lady Michelle Obama offered a personal perspective on why her husband should be re-elected, telling the convention that the same values she fell in love with guide him each day in the White House.

"In the end, for Barack, these issues aren't political -- they're personal," Mrs. Obama said, adding: "Barack knows the American Dream because he's lived it, and he wants everyone, everyone in this country to have the same opportunity, no matter who we are, or where we're from, or what we look like, or who we love."

Health and Human Services Secretary Kathleen Sebelius defended the 2010 health care law despised by Republicans, saying the changes that prevent insurance companies from denying coverage for pre-existing conditions and allow parents to keep their children on family policies up to age 26 were "what change looks like."

She said Romney and GOP running mate Rep. Paul Ryan want to repeal the health care law and effectively end the Medicare program for senior citizens by making it a voucher program.

"Mitt Romney, quite simply, doesn't get it," declared San Antonio Mayor Julian Castro in his keynote address, later adding: "We know that in our free-market economy, some will prosper more than others. What we don't accept is the idea that some folks won't even get a chance."

Romney and Republicans "are perfectly comfortable with that America -- in fact, that's exactly what they're promising us," Castro said.

Massachusetts Gov. Deval Patrick earlier challenged Obama supporters to be more forceful in supporting the president's record in the face of unrelenting Republican attempts to discredit the administration's accomplishments, such as ending the Iraq war and delivering "the security of health care to every single American in every single corner of the country."

"It's time for Democrats to grow a backbone and stand up for what we believe," Patrick said to cheers..

Other speakers mocked Romney's overseas financial holdings -- a Swiss bank account, a Cayman Islands tax shelter -- revealed in the two years of tax returns the former Massachusetts governor has released so far. Senate Majority Leader Harry Reid challenged Romney to make public more of his tax records, citing the example of the 12 years of returns Romney's father, George, made public in his 1968 presidential run.

"Trust comes from transparency, and Mitt Romney comes up short on both," Reid said.

In a video played in the Time Warner Cable Arena where the convention opened, former President Jimmy Carter said Obama prioritized the middle class in making the difficult decisions required of the nation's highest office.

"He has done it all in the face of bitter, unyielding, in fact unprecedented partisan opposition," Carter said in reference to congressional Republicans.

The convention will conclude Thursday with Obama accepting his party's nomination exactly two months before the November 6 election. Former President Bill Clinton will address the gathering on Wednesday night.

Newark Mayor Corey Booker energized Tuesday's first session of the convention gaveled to order Tuesday afternoon with a rousing call for delegates to pass the party platform that represented "our fundamental national aspiration."

To applause and cheers, Booker said voters in November faced a choice between GOP policies offering either "a country of savage disparity favoring the fortunate few" or the Democratic goal of equal opportunity for all.

"This is our American mission," he added.

Later, Chicago Mayor Rahm Emanuel recalled the litany of crises that greeted the Obama administration when it assumed power in January 2009 -- a Wall Street meltdown, economic recession, wars in Iraq and Afghanistan, a collapsing auto industry.

"Each crisis was so deep, and so dangerous, any one of them would have defined another presidency," said Emanuel, who was Obama's chief of staff. "We faced a once-in-generation moment in American history, and fortunately, we have a once-in-a generation president."

Other speakers told how Obama's decision to bail out the struggling auto industry saved vital jobs and kept a mainstay of the U.S. economy afloat. The goal was to contrast a politically unpopular decision -- spending tax dollars to help a private industry -- with Romney's call at the time to let Detroit go bankrupt. Romney now says he advocated a managed bankruptcy similar to the eventual result under Obama.

Senior campaign officials told CNN that the convention has three main objectives -- to outline the clear choice facing voters, to highlight Obama's leadership in championing necessary but politically unpopular steps such as health care reform and the auto industry bailout, and to present a detailed plan for creating jobs for the middle class.