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The accomplished musician all but confirmed his entry into the race on Friday, releasing a YouTube video where he plays the presidential anthem Hail to the Chief and then nods.
Mr O'Malley is the third entrant for the Democrats, and will face Vermont Senator Bernie Sanders and prohibitive favourite Hillary Clinton for the nomination.
There's a certain amount of irony in Mr O'Malley's decision to challenge the former secretary of state. As Lisa Lerer and Ken Thomas of the Associated Press point out, the Clintons assisted in the governor's rise to prominence, with Bill Clinton once saying that he wouldn't be surprised if Mr O'Malley eventually became president.
"When O'Malley faced a tough race for governor in 2006, the Clintons held fundraisers and starred at rallies for him," they write. "In the final days of that race, Bill Clinton answered pleas from O'Malley aides to appear in a campaign ad - stopping in an airport to tape an endorsement of his 'good friend'."
Now, however, Mrs Clinton and Mr O'Malley are political adversaries - albeit on a very tilted playing field. According to the latest polls, Mrs Clinton has the support of nearly 60% of Democratic primary voters, while Mr O'Malley barely registers.
The former governor is well credentialed, writes the New Republic's Brian Beutler, but it's difficult to see how he can gain much traction against the Clinton juggernaut.
"In any party, there's usually an appetite for an outsider to challenge the establishment, but this year there doesn't seem to be much hunger for a different establishmentarian to supersede the frontrunner," he writes.
And then there's the fact that Mr O'Malley has had one of the more rocky run-ups to a presidential announcement in recent memory.
First, last fall, a little-known Republican candidate beat his hand-picked successor to the Maryland governorship in what was generally considered the biggest upset of the mid-term elections.
Then, in April, Baltimore erupted in racial unrest following the death of Freddie Gray while in police custody. Practically overnight what had been a bullet point on Mr O'Malley's resume - his successful crackdown on violent crime while the city's mayor - became a possible liability.
Up until then Mr O'Malley had touted his "zero tolerance" policy of prosecuting smaller violations of the law in order to prevent larger ones. In recent days, however, critics have said that this policy encouraged overly aggressive law-enforcement tactics that helped contribute to the toxic distrust between the police and Baltimore's black community.
"O'Malley fashions himself as a no-nonsense crime fighter, but his city has not only become a national monument to urban devastation, but it is experiencing one of its worst murder sprees in years," writes McClatchy's David Lightman.
A Baltimore civil rights group called Baltimore Bloc has said that it will demonstrate during Mr O'Malley's Saturday announcement in a park overlooking the city's downtown.
"O'Malley didn't do nothing for Baltimore," Duane Davis, one of the protest organisers, told the Baltimore Sun. "He created a prison system in Maryland. That's his legacy."
If there is any space to challenge the former secretary of state, it's likely to be on her political left - an area Mr Sanders, a self-professed socialist focusing on income inequality, is trying to exploit.
According to Fivethirtyeight.com's Harry Enten, however, Mrs Clinton actually rates as more liberal than Mr O'Malley.
While Mr O'Malley does have a solid record of liberal accomplishments as Maryland governor, he's never been seen as a ideological true believer.
"It's not just that he's a notoriously leaden public speaker; it's that, as progressive as his governing record is, he's oddly reluctant to champion liberal values in the terms many on the left crave," writes Slate's Alec MacGillis. "He is, by his own account, not a tribune but a technocrat, not an orator but a doer."
Nevertheless, HA Goodman lays out the case for Mr O'Malley's candidacy in the opinion section of Friday's Baltimore Sun.
"The former Baltimore mayor and two-term governor offers a genuine alternative to the status quo within Washington and a real threat to any GOP challenger - especially Jeb Bush," he writes. "Mr O'Malley, unlike Hillary Clinton and Jeb Bush, isn't linked to perpetual scandal and criticism, nor is he beholden to foreign donors, investment banks or a family surname."
The former governor, he writes, has consistently been on the right side of progressive issues - such as gay marriage, marijuana decriminalisation, international trade agreements and the Iraq War - where Mrs Clinton has waffled.
Mr O'Malley has himself attempted to differentiate his candidacy from that of Mrs Clinton's - both by emphasising his relative youth (52 versus 67) and the baggage that comes with the Clinton last name.
"Let's be honest here, the presidency of the United States is not some crown to be passed between two families - it is an awesome and sacred trust to be earned and exercised on behalf of the American people," he said in March.
At least so far, however, rank-and-file Democrats seem satisfied with the prospect of another Clinton occupying the Oval Office.
Just as beauty pageants designate a runner-up "in case the winner can't fulfil her duties", Mr O'Malley's greatest appeal to Democrats may be as a back-up plan if Mrs Clinton's campaign were to catastrophically implode.
"He's a legitimate national candidate," Democratic strategist Scott Ferson told the Hill, a Washington political newspaper. "If Hillary for some reason doesn't become inevitable, some candidate will have a shot to step in, and he could be that person."
In the history of US politics, stranger things have happened. And Mrs Clinton's image has taken a number of hits recently, following questions about donations to her family's non-profit foundation and her use of a private email server while secretary of state.
If Mr O'Malley is going to fulfil the presidential prediction Mr Clinton made many years ago, however, a great many developments are going to have to break his way. And, at least so far, fortune hasn't done him any favours.
Output:
On Saturday Martin O'Malley, former governor of Maryland and mayor of Baltimore, is expected to announce his bid for the 2016 Democratic presidential nomination.