But it seemed unlikely that anything that happened in Vegas would fundamentally change the dynamics of a race in which Hillary Rodham Clinton remains the front-runner, and Sen. Bernie Sanders (I-Vt.) is providing a surprisingly strong challenge from Clinton's left.
Dante Scala, a political science professor at the University of New Hampshire, said O'Malley "acquitted himself well" but did not have the same kind of breakthrough performance that Carly Fiorina, the former Hewlett Packard executive, did in the Republican debates.
"O'Malley needs one soon," said Scala. "That wasn't it last night, but it was a start."
O'Malley partisans said they were quite pleased with the former governor's performance, suggesting he looked presidential and provided forceful responses on many issues, including gun control — one area where Sanders has a less progressive record than his rivals. O'Malley, by contrast, named the National Rifle Association when asked for his biggest enemy during the debate.
Hoping to build on that momentum, O'Malley scheduled an afternoon event here Wednesday with parents who lost a daughter in the Aurora, Colo. shootings.
At a minimum, O'Malley boosters said, he showed Tuesday that he belongs on the same stage as Clinton and Sanders — not a common sentiment regarding the other two Democratic hopefuls who participated, former Virginia senator Jim Webb and former Rhode Island senator and governor Lincoln Chafee.
"Yesterday was the kickoff to the campaign season, and Martin O'Malley is firmly entrenched on the playing field," said Terry Lierman, a former chairman of the Maryland Democratic Party who is serving as O'Malley's campaign treasurer.
With less than four months before the first nominating contests in Iowa and New Hampshire, others argued that O'Malley needed to do more than just be on the field — that he needed to score a few touchdowns on memorable plays.
"The coverage needed to be, 'Martin O'Malley wins the debate,'" said one former high-ranking Iowa Democratic Party official, who requested anonymity to speak more candidly. "While it wasn't a bad performance, it wasn't enough."
That sentiment was echoed by Dennis Goldford, a political science professor at Drake University in Iowa, who said he thinks O'Malley "remains an asterisk after this."
"He came across as very well intentioned and bright — but ineffectual," Goldford said, adding that it is still hard to see what is O'Malley's niche in the race.
O'Malley strategist Bill Hyers argued his candidate still has plenty of time to assert his place in the contest.
"The majority of primary voters frankly don't know who Governor O'Malley is, but last night, in 17 minutes of primetime, he succeeded in making a forceful first impression by introducing his record of principled leadership and progressive results," Hyers said in a statement.
O'Malley has invested a great deal of time in Iowa, where he is hoping for a surprisingly strong finish in the first-in-the-nation caucuses. But he has remained stuck in the single digits in polls and does no better in New Hampshire, home of the first primary.
O'Malley has also been struggling to raise the kind of money that will be necessary to compete against Clinton and Sanders. Both those Democrats released fundraising totals for the previous quarter on Sept. 30, the quarter's final day. The Clinton campaign said she had raised about $28 million, while Sanders put his haul at more than $26 million.
O'Malley still has not disclosed a figure, leading to speculation that it is embarrassingly low.
Among other things, O'Malley aides hoped that a strong debate performance could be parlayed into a fresh wave of donations. As of Wednesday morning, it was unclear to what extent O'Malley would capitalize.
Sanders said that he had raised $1.3 million online in the four hours after the debate began. O'Malley officials wouldn't discuss their fundraising efforts with any specificity.
"I think (the debate) will not only provide a financial bump, it will provide a bump in the polls," Lierman said.
During the debate, O'Malley sought to portray himself as a strong executive with a record of pushing through progressive causes that others have only talked about. As governor of Maryland, he championed measures raising the minimum wage, legalizing same-sex marriage, repealing the death penalty, expanding gun control and allowing undocumented immigrants to attend college at in-state rates.
In an otherwise gaffe-free performance on Tuesday, O'Malley mixed up the names of the leaders of Syria and Russia, a mistake that perhaps reinforced his relatively light record on foreign affairs.
Joe Trippi, a veteran Democratic strategist, said he thought O'Malley had a good debate overall.
"But I don't know that he had a moment that everyone's talking about or will change the dynamic," he said.
Ultimately, Trippi said, O'Malley's fate will be determined by whether he can continue a "slow grind" toward the Iowa caucuses and surprise people with his performance.
"I've seen that movie before, but that doesn't necessarily mean it's going to happen here," Trippi said.