Martin O'Malley speaks at the Congressional Hispanic Caucus Institute conference in Washington on Oct. 7. (Andrew Harrer/Bloomberg)

After complaining loudly for months about the scarcity of Democratic debates, former Maryland governor Martin O’Malley faces a put-up-or-shut-up moment Tuesday.

The presidential hopeful will take the debate stage in Las Vegas with one overriding objective: proving that he is a credible alternative to the two leading contenders for the 2016 Democratic presidential nomination.

When he launched his campaign in May, O’Malley fashioned himself as a more progressive alternative to Hillary Rodham Clinton — a space that Sen. Bernie Sanders (I-Vt.) has since occupied with a gusto that no one anticipated.

“I think right now most people in the Democratic Party think they only have two choices,” O’Malley acknowledged during an interview. He said the party’s debates would be “make-or-break moments for every campaign,” including his.

Here's a look at one of the lesser-known candidates who will be onstage for the Democratic presidential debate Oct. 13. (Osman Malik and Sarah Parnass/The Washington Post)

Associates say that O’Malley, who lags far behind Clinton and Sanders in the polls, has been huddling with campaign aides and longtime advisers to prepare for a two-hour debate on CNN in which he will present himself as a progressive with a record of getting things done during his tenures as Maryland’s governor and Baltimore’s mayor.

Two other Democratic candidates — former U.S. senator Jim Webb of Virginia and former Rhode Island governor and U.S. senator Lincoln Chafee — will also share the stage.

Analysts say that O’Malley needs to have a standout performance and make the case that he would be more electable in November 2016 than either Clinton, whose trust among voters has been eroded by continuing questions about her e-mail practices as secretary of state, and Sanders, a self-described democratic socialist whose views may be too far to the left for general-election voters.

“If all three of them give a credible performance, it’s not going to propel O’Malley forward,” said Craig Varoga, a veteran Democratic strategist who advised O’Malley during his 2010 reelection campaign for governor but is no longer working for him.

Varoga said he is skeptical that the debate will prove a game-changer for O’Malley, in part because the overarching narrative of the Democratic contest has already been set as a contest between Clinton, the wobbling front-runner, and Sanders, the insurgent challenger. What would be more likely to change that dynamic would be a late entry into the race by Vice President Biden, Varoga said.

Martin O'Malley speaks to members of the media at the Congressional Hispanic Caucus Institute conference in Washington on Oct. 7. (Andrew Harrer/Bloomberg)

For weeks, O’Malley has been clamoring for more debates than the six that have been sanctioned by the Democratic National Committee. He argues that the schedule was designed to “circle the wagons” around Clinton by limiting her exposure to challengers.

O’Malley’s constant cry of “more debates” has only raised the bar that he will have to meet to walk away a beneficiary of the first encounter. This week his campaign released a photo of him, via Snapchat, reading a briefing on his iPad as he was working out in the gym.

“Not to set expectations too high, but I think a lot is riding on these first few debates,” said H. Boyd Brown, a former state legislator and a leading O’Malley supporter in South Carolina, an early primary state. “That’s a lot of his strategy right now. It’s been, ‘Let’s get to the debates and prove our salt.’ ”

O’Malley advisers say the debates will offer the former governor his largest audience to date in the campaign and a chance to make a strong impression among people who aren’t familiar with him.

“I feel like the campaign really begins on Oct. 13 in a lot of ways,” O’Malley said. “It’s malpractice as a party to have waited so long to begin our debates. Eight years ago, we had already had nine debates. But now we’ll finally have our first, and finally we’ll get the opportunity to make our case to the American people.”

The downside for O’Malley — after having built up the debates as much as he has — is if he tanks. “If he gives a mediocre performance, he’s done,” said a Democratic operative with ties to O’Malley who requested anonymity to speak more freely.

Since many who watch Tuesday will be seeing O’Malley for the first time, some advisers say he should keep things positive, with few jabs aimed at his opponents.

On the other hand, O’Malley has demonstrated in recent days that he is not afraid to take aim at the leading candidates. In an interview with a radio station in Concord, N.H., O’Malley accused Sanders of having views on gun control that are “different than the mainstream of the Democratic Party.”

A few days later, on MSNBC, O’Malley made a not-too-thinly-veiled reference to Clinton’s ­recent change in positions on ­several issues, including the ­12-nation Pacific Rim trade deal negotiated by the Obama administration. “Unlike the weather vane that blows in the wind, I know where I stand,” O’Malley told host Chris Hayes.

Some analysts argue that O’Malley’s fate could turn as much on Sanders’s performance on Tuesday as his own.

There’s a real question as to “how plausible Sanders will look as a presidential candidate to a lot of people tuning in for the first time,” said Dante Scala, a professor of political science at the University of New Hampshire. If Sanders doesn’t meet that test, he said, some voters could begin a fresh search for an alternative to Clinton.

O’Malley is no stranger to the debate setting, although it’s been a few years since he tangled with his last adversary, former Maryland governor Robert L. Ehrlich Jr. (R).

Todd Eberly, a professor of political science at St. Mary’s College of Maryland, said O’Malley proved an able debater when he stuck to policy issues. But he has a tendency to overreach and use “flowery language,” Eberly said.

“Does he show up as the solid progressive policy wonk or does he try to be the next JFK?” Eberly asked. “He’s much better when he sticks to policy.”