Baltimore (CNN) Martin O'Malley launched his presidential campaign Saturday with an appeal to the party's progressive base that he hopes will upend the conventional wisdom that Hillary Clinton is destined to clinch the Democratic nomination.

The former Maryland governor unveiled his campaign in Baltimore, the city where he was once mayor -- a role that is central to his political persona. But his Baltimore credentials could become more of a challenge than he initially thought after a riot erupted in the city in April.

Speaking in rolled-up sleeves at Federal Hill Park, O'Malley began with a call for economic fairness and closing the gap between rich and poor in America.

"This is the urgent work calling us forward today: to rebuild the truth of the American Dream for all Americans," O'Malley said. "And to begin right now."

He touched upon last month's unrest in Baltimore, saying the aftermath of Freddie Gray's death was about more than race or policing in America, but about "the scourge of hopelessness" in the nation's cities.

"There is something to be learned from that night, and there is something to be offered to our country from those flames," he said.

A small group of protesters tried to interrupt his speech, shouting remarks like "black lives matter" and blowing on whistles. As O'Malley discussed the problem of income inequality and concentration of wealth, one protester angrily yelled out: "You did that! It was you!"

But the protesters largely failed to be a disruptive presence at the launch event, where O'Malley's supporters, numbering in the hundreds, reacted enthusiastically to his speech.

O'Malley also took a shot at Clinton and Republican Jeb Bush, using an attack on Goldman Sachs to suggest they were too close to Wall Street to be advocates for the less fortunate in America.

"Recently, the CEO of Goldman Saches let his employees know that he'd be just fine with either Bush or Clinton. I bet he would," O'Malley said. "Well, I've got news for the bullies of Wall Street: The presidency is not a crown to be passed back and forth by you between two royal families. It is a sacred trust to be earned from the people of the United States, and exercised on behalf of the people of the United States."

Taking on Hillary

Perhaps O'Malley's biggest challenge is finding a way to dent the Clinton political machine while also proving that he's a competitive candidate in his own right -- not just a backup for progressives who would rather see Sen. Elizabeth Warren of Massachusetts in the White House. A Quinnipiac University poll released this week places Clinton 56 percentage points ahead of O'Malley.

Saturday's announcement was not a surprise. Over the past year, the 52-year-old traveled repeatedly to the early nominating states of Iowa, New Hampshire and South Carolina to spark voters' attention to his likely bid.

On the road, O'Malley touts a string of progressive actions he oversaw as governor of Maryland. Under his leadership, the state tightened gun laws, implemented a progressive tax code and legalized same-sex marriage. He also expanded the state's health care rolls, championed Obamacare and signed a bill raising the state's minimum wage to $10.10 an hour.

Yet O'Malley found himself defending that Maryland record recently when riots broke out in Baltimore over the death of a 25-year-old African-American man under police custody.

Photos: Moments from Martin O'Malley's career Photos: Moments from Martin O'Malley's career Former Democratic Maryland Governor Martin O'Malley addresses the 2015 International Association of Fire Fighters on March 10. Hide Caption 1 of 11 Photos: Moments from Martin O'Malley's career O'Malley has not been shy about potentially running in 2016, saying about potential competitors Jeb Bush and Hillary Clinton that the presidency is not a "crown" to be passed through families. Hide Caption 2 of 11 Photos: Moments from Martin O'Malley's career In some early 2016 polling, O'Malley is pulling single-digit numbers. Hide Caption 3 of 11 Photos: Moments from Martin O'Malley's career O'Malley speaks during the 2015 Alfred K. Whitehead Legislative Conference and Presidential Forum on March 10. Hide Caption 4 of 11 Photos: Moments from Martin O'Malley's career O'Malley speaks at a Democratic fundraiser at Gibson's Bookstore and True Brew Cafe on March 6 in Concord, New Hampshire. Hide Caption 5 of 11 Photos: Moments from Martin O'Malley's career O'Malley speaks during a ceremony commemorating the bicentennial of the writing of The Star-Spangled Banner at Fort McHenry National Historic Park on September 13, 2014. Hide Caption 6 of 11 Photos: Moments from Martin O'Malley's career O'Malley delivers remarks before Barack Obama takes the stage at a Costco store January 29, 2014 in Lanham, Maryland. Hide Caption 7 of 11 Photos: Moments from Martin O'Malley's career O'Malley addresses a conference commemorating the 10th anniversary of the Center for American Progress on October 24, 2013. Hide Caption 8 of 11 Photos: Moments from Martin O'Malley's career O'Malley speaks during the National Clean Energy Summit 6.0 on August 13, 2013 in Las Vegas. Hide Caption 9 of 11 Photos: Moments from Martin O'Malley's career O'Malley and his wife Katie O'Malley arrive for a State Dinner in honor of British Prime Minister David Cameron at the White House on March 14, 2012. Hide Caption 10 of 11 Photos: Moments from Martin O'Malley's career O'Malley campaigns for reelection on October 27, 2010, in Frederick, Maryland. Hide Caption 11 of 11

The treatment of Freddie Gray, which sparked a national dialogue about police conduct toward racial minorities, drew renewed scrutiny to the controversial zero-tolerance policing strategy that O'Malley advocated for as mayor -- part of an aggressive strategy to crack down on crime.

In an interview with CNN's Jake Tapper last month, O'Malley declared that Baltimore saw a "record reduction in violent crime" under his watch.

There are "probably now 1,000 mostly young, poor African-American men who did not die violent deaths in our city" because of these policies, O'Malley said

Photos: Who's running for president? Ted Cruz, Donald Trump, John Kasich, Hillary Clinton and Bernie Sanders, Hide Caption 1 of 6 Photos: Who's running for president?



"So, ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump told the crowd at his announcement. Businessman Donald Trump announced June 16 at his Trump Tower in New York City that he is seeking the Republican presidential nomination. This ends more than two decades of flirting with the idea of running for the White House."So, ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump told the crowd at his announcement. Hide Caption 2 of 6 Photos: Who's running for president?



"These are all of our stories," Cruz told the audience at Liberty University in Virginia. "These are who we are as Americans. And yet for so many Americans, the promise of America seems more and more distant." Sen. Ted Cruz of Texas has made a name for himself in the Senate, solidifying his brand as a conservative firebrand willing to take on the GOP's establishment. He announced he was seeking the Republican presidential nomination in a speech on March 23."These are all of our stories," Cruz told the audience at Liberty University in Virginia. "These are who we are as Americans. And yet for so many Americans, the promise of America seems more and more distant." Hide Caption 3 of 6 Photos: Who's running for president? Ohio Gov. John Kasich joined the Republican field July 21 as he formally announced his White House bid.



"I am here to ask you for your prayers, for your support ... because I have decided to run for president of the United States," Kasich told his kickoff rally at the Ohio State University. Hide Caption 4 of 6 Photos: Who's running for president?



"Everyday Americans need a champion, and I want to be that champion -- so you can do more than just get by -- you can get ahead. And stay ahead," she said in her announcement video. "Because when families are strong, America is strong. So I'm hitting the road to earn your vote, because it's your time. And I hope you'll join me on this journey." Hillary Clinton launched her presidential bid on April 12 through a video message on social media. The former first lady, senator and secretary of state is considered the front-runner among possible Democratic candidates."Everyday Americans need a champion, and I want to be that champion -- so you can do more than just get by -- you can get ahead. And stay ahead," she said in her announcement video. "Because when families are strong, America is strong. So I'm hitting the road to earn your vote, because it's your time. And I hope you'll join me on this journey." Hide Caption 5 of 6 Photos: Who's running for president?



"This great nation and its government belong to all of the people and not to a handful of billionaires, their super PACs and their lobbyists," Sanders said at a rally in Vermont on May 26. Sen. Bernie Sanders , an independent from Vermont who caucuses with Democrats, announced his run in an email to supporters on April 30. He has said the United States needs a "political revolution" of working-class Americans to take back control of the government from billionaires."This great nation and its government belong to all of the people and not to a handful of billionaires, their super PACs and their lobbyists," Sanders said at a rally in Vermont on May 26. Hide Caption 6 of 6

O'Malley may also face questions about his popularity in his home state. His standing took a hit last year when Maryland voters rejected his handpicked successor in the governor's race, Democrat Anthony Brown. Brown lost the statehouse to Republican Larry Hogan.

"I can tell you my feelings were hurt," O'Malley said about the loss. "We had done a lot of really good things in Maryland, and in the end you did not hear much about it during the campaign."

But he added, "I was not on the ballot."

A fresh voice for the party

O'Malley is aiming to present himself as a fresh voice for the party -- one who speaks for a different generation than Washington heavy hitters such as the 67-year-old Clinton. The former governor plays guitar in his Celtic rock band O'Malley's March, and at some gigs he has occasionally bared his biceps in sleeveless shirts.

JUST WATCHED O'Malley flashback: The 2007 Clinton endorsement Replay More Videos ... MUST WATCH O'Malley flashback: The 2007 Clinton endorsement 01:15

Beyond Clinton and O'Malley, Bernie Sanders is the only other Democrat who has announced a 2016 presidential bid. Sanders is also popular among liberals and garnered 15% in the Quinnipiac poll.

O'Malley has remained optimistic about his own prospects, telling CNN in March he could turn around his low numbers by outworking the competition.

"When you start off as potential candidate for president and your name recognition is low, you have to just go from county to county, from town to town and engage people in order to change that around," O'Malley said then. "I guess another way to say it is this: Look, it is not unusual for there to be an inevitable frontrunner early in a contest who has fantastic name recognition, and is therefore inevitable right up until he or she is no longer inevitable."