He's got some challenges Tuesday night. (Pablo Martinez Monsivais/AP)

LAS VEGAS — Sen. Bernie Sanders (I-Vt.) spent much of Monday at an undisclosed location in Nevada preparing for his biggest moment on the national political stage.

Tuesday night’s Democratic presidential debate offers Sanders a long-awaited chance to go toe-to-toe with Hillary Rodham Clinton and his biggest opportunity yet to take his message to a national audience. But the two-hour encounter on CNN poses as many potential risks as it does rewards.

While Sanders has been drawing massive crowds on the campaign trail, many Democratic voters still don’t know much about him and some aren’t sure what to make of the “democratic socialist” label the 74-year-old senator attaches to himself.

Sanders’s costly agenda is bound to come under scrutiny, and he’s likely to get pressed on one issue where he hasn’t necessarily been more progressive than most of his primary-season rivals: gun control.

Here’s a look at some of Sanders’s potential vulnerabilities:

The electability problem. Though he has surged in polling among Democrats in early-nominating states, Sanders still faces the same basic question as he did when he first got in the race: Is the country ready to elect a self-described socialist as president?

The senator from Vermont has acknowledged that he needs to explain what the terms means and doesn’t mean to people just getting to know him. On Tuesday night, that will include millions of television viewers.

“Does anyone here think I’m a strong adherent of the North Korean form of government? That I want all of you to be wearing similar-colored pajamas?” Sanders said jokingly to a group of college students recently in New Hampshire when asked about the topic. (The answer to both questions was no.)

What Sanders points to instead are the policies of governments in Scandinavia and elsewhere that include far more generous health-care and retirement benefits, along with guaranteed sick time and family leave for workers.

While that may strike many Democratic voters as reasonable, Sanders’s discussion of his philosophy has produced some moments that might not go over as well.

Sen. Bernie Sanders (I-Vt.), a White House contender in 2016, is known for his stances on budget issues and war. Here are his takes on Obamacare, Social Security and more. (Julie Percha/The Washington Post)

During an appearance Sunday on NBC’s  “Meet the Press,” for example, host Chuck Todd posed this question: “Are you a capitalist?”

“No,” Sanders responded. “I’m a democratic socialist."

Sanders could also face questions Tuesday about his limited appeal at least so far to black and Latino voters, key constituencies in the Democratic Party. A new CNN-ORC poll Monday showed Sanders with only 4 percent backing among African Americans in South Carolina, the first primary state in the South.

The gun-control problem. On many issues, including gay rights and trade, Sanders arrived at his progressive positions well before his Democratic rivals. When it comes to guns, he is playing catchup.

Sanders, who represents a state with a deep hunting tradition and little appetite for gun control, has a mixed record on the issue. As a member of the House in 1993, he voted against the landmark Brady bill, which mandated federal background checks on firearms purchasers. He also voted in 2005 to shield manufacturers from lawsuits brought by victims of gun violence.

Sanders’s voting record includes more-recent stands in favor of gun-control measures, and as a presidential candidate he has pledged to develop a comprehensive package of reforms to stem violence, including stronger background checks for gun purchases and “a revolution” in the way the country treats mental illness.

Still, there’s plenty of room for contrast here, particularly in the aftermath of several high-profile mass shootings in recent months. Clinton is pushing the case for reforms hard, as is former Maryland governor Martin O’Malley, who shepherded a sweeping package of gun-control measures though his state’s legislature in 2013.

The free-tuition-for-rich-kids problem. One of Sanders’s most popular proposals on the campaign trail is his plan to make college tuition free at public colleges and universities.

Clinton, however, has her own college affordability plan and has already shown a willingness to spar with Sanders on this issue. The Clinton “compact” calls on families “to do their part” on college costs but seeks to dramatically reduce the debt that students incur.

“I am not going to give, you know, free college to wealthy kids,” Clinton said during a recent appearance before the Des Moines Register’s editorial board.

Sanders has defended his plan as more simple and straightforward. He also acknowledges his plan would be expensive about $70 billion a year and explaining how he would pay for his agenda is another potential vulnerability for him Tuesday night.

Sanders has a funding source for his tuition plan — a tax on Wall Street transactions by investment houses, hedge funds and other speculators — but some question whether it would generate enough money.

The presentation problem. Sanders certainly knows how to address a rally. His crowds stay transfixed despite a stump speech that clocks in at close to an hour. But he will be in a different element Tuesday night, and his record as a debater isn't quite as strong.

Some say Sanders can come across as grumpy and gruff.

John MacGovern, a Republican who debated Sanders the 2012 Senate race, told CNN that Sanders can "be very aggressive, very strong, and he speaks loud and in a rat, tat, tat, tat, drum beat."

On the other hand, Sanders has been a frequent guest on talk shows as of late and demonstrated a growing ability to answer questions succinctly — a skill that could serve him well on Tuesday.

The other-guys-on-stage problem. In his ideal world, Sanders would get a shot at Clinton in a one-on-one debate. But there will be five lecterns on the stage Tuesday, with at least three of them occupied by candidates — O’Malley, former senator Jim Webb of Virginia and former senator and governor Lincoln Chafee of Rhode island — eager to shake up the race.

That fact makes it more challenging for Sanders to draw a contrast with Clinton. And it could leave Sanders open to more criticism than he would get in a one-on-one face-off.