Clinton wished O'Malley well in an address late Monday, calling him "a great public servant who has served Maryland and our country."
The former Baltimore mayor, who had been rumored to be considering a presidential run for years, oversaw an issues-based campaign that was heavy on retail politics in Iowa and New Hampshire; he spent more time in Iowa last year than either Clinton or Sanders. Even his some of his critics have given him credit for the disciplined campaign.
But political analysts say O'Malley's effort was severely hampered by timing, including the decision by Sanders to enter the race early. The Vermont senator managed to coalesce the same anti-Clinton voters that O'Malley had hoped to court. The governor also struggled to capture attention in a media landscape dominated by Republican Donald Trump.
"From the moment Governor O'Malley entered this race, he campaigned with heart and with a singular focus on building a better future for American families," Democratic National Committee chairwoman Debbie Wasserman Schultz said in a statement. "Gov. O'Malley knows that progress is not inevitable — progress is a choice, and he has the record to show it."
O'Malley and Wasserman Schultz had exchanged terse words throughout much of the campaign after O'Malley repeatedly harped on party leaders for sanctioning only six debates before Iowa and New Hampshire. The Democratic candidates — now, minus O'Malley — will hold a seventh debate on Thursday.
The former Maryland governor influenced the race in other ways, as well. He was the first candidate to call on the U.S. to accept more refugees from Syria, for instance — an idea that was later adopted by Clinton. And his campaign released detailed policy memos on immigration, Wall Street reform and gun control before any of the other candidates.
O'Malley always knew he would be in for a serious challenge running against Clinton, a onetime ally with strong support in the party. The governor tried to sell voters on a more liberal approach, one based on his final years in Annapolis and accomplishments that included a same-sex marriage law and a higher minimum wage.
But while O'Malley's campaign was technically smooth, outside forces repeatedly delivered setbacks. The rioting that took place in Baltimore in April came at a time when O'Malley was trying to pitch himself as a technocrat who had turned the city around. Earlier, his lieutenant governor, Anthony Brown, lost to Republican Larry Hogan in last year's gubernatorial election.
By last fall O'Malley was struggling to change the narrative that the contest for the Democratic nomination increasingly appeared to be a two-person race. And by early December, in an indication of his inability to capture support, O'Malley was forced to take out a $500,000 loan just to keep his campaign afloat.
On Monday night, sounding a recently developed campaign theme, O'Malley urged his supporters to "hold strong" to the issues they had been pushing for months.
"In conclusion, there is no conclusion," O'Malley said. "Thank you for allowing me to make this offering out of love."