Democratic debate: What Clinton, Sanders, and the rest need to prove

By most accounts, Tuesday night's first Democratic presidential debate will be a far cry from it's two Republican predecessors.

There will be no reality-TV star, no breakthrough outsider candidates, and no veritable army of contenders on an overflowing debate stage. It probably won't be a slugfest, and it probably won't attract 25 million voters, as the first Republican debate did.

Instead, viewers can expect substantive debate on serious issues like gun control, trade, banks, and foreign policy. Nonetheless, each candidate has something to prove – honesty and authenticity for Hillary Clinton, electability for Bernie Sanders, viability for the rest – and will jockey for air time to make their case to Americans.

Here's what to expect in the first Democratic 2016 presidential debate, which begins at 8:30 p.m. Eastern on CNN, in Las Vegas.

Hillary Clinton: Perhaps no one has more to prove – or lose – than the beleaguered, presumed frontrunner who entered the 2016 presidential race as the party favorite. She has fallen in the polls and dashed Democratic expectations ever since.

For Mrs. Clinton, an experienced and aggressive debater who participated in more than 20 debates in the 2007-2008 election cycle, Tuesday night's debate will be an opportunity to distinguish herself from the pack and demonstrate that she is electable, especially in contrast to Bernie "I'm not a capitalist" Sanders. In terms of sheer debate prowess, she will likely shine.

If, or likely, when, the elephant, or rather donkey, in the room – namely Clinton's email controversy – rears its head, expect her to pivot and deflect. After a brief defense, she may use the opportunity to refocus on why she's running for president, what policies she's proposed, and what she would do as commander in chief.

There's also a possibility she'll refer to recent developments that may feed arguments that the investigation into the deadly 2012 attack on the US consulate in Benghazi, Libya, is a partisan attack: House Majority Leader Kevin McCarthy's recent comments that a House committee's Benghazi investigation was worthwhile because it damaged Clinton in the polls. And a fired committee staff member's accusations that the panel is focused on a partisan investigation of Clinton rather than an objective consideration of the attack.

Bernie Sanders: The Senator from Vermont has so far outperformed expectations, even surpassing Clinton in the polls in some early primary states like New Hampshire and Iowa. But the debate may prove to be a tough reckoning for the self-described socialist. He picked an inopportune moment – the Sunday before the debate – to tell host Chuck Todd of NBC's "Meet the Press" that he's a Democratic socialist, not a capitalist. As NBC News pointed out, a socialist running for president is more unpopular than a Muslim, atheist, or gay candidate, with a full 50 percent of respondents telling Gallup they would not vote for a socialist candidate.

What's more, only 26 percent of registered voters identify as liberal according to a Wall Street Journal analysis of national polls, so Mr. Sanders will need to use the debate to reach out to moderates. And Sanders tends to do well on the trail when he's speaking to enthusiastic, liberal supporters who want to hear his message on economic inequality, a theme from which he rarely deviates. He won't have that kind of audience at the debate, and he'll have to keep his energy and momentum up on topics other than economic inequality, or risk coming off as dry and preachy. As Pat Rynard, a former Democratic campaign staffer from Iowa, told USA Today, “[D]ebates tend to favor interaction and candidates quick on their toes who can give punchy responses.”

Martin O'Malley: Despite campaigning longer and harder than most of his rivals, the former governor of Maryland is struggling to raise money and polling low – a new Washington Post poll has him at just 4 percent in his home state of Maryland.

No one recognizes this more than Mr. O'Malley himself.

"It's a very, very important opportunity for me to not only present my vision for where the country should head, but also 15 years of executive experience, actually accomplishing the progressive things some of the other candidates can only talk about," he said ahead of the debate.

More than most, he needs a breakthrough moment to catch interest and attention. Expect him to take swings at his rivals, especially Clinton, to get air time.

As Stephanie Cutter, a Democratic strategist and top staffer in the President Obama’s 2012 re-election campaign, told USA Today, “This is the last best chance for Martin O’Malley.”

Lincoln Chafee and Jim Webb: Mr. Webb, a one-term senator from Virginia, and Mr. Chafee, who served both as a Rhode Island senator and governor, are both hanging on by their fingernails, with polling, name recognition, and campaigning near nil. Few know who they are or why they're in the race, so the debate will help them answer those basic questions and introduce themselves to voters. Their best chance is having a Carly Fiorina moment: an opportunity to break out from the shadows and make themselves known.

Joe Biden: He's one of the biggest names of the night, even though he's not participating in the debate (barring any 11th-hour declarations). Fans have been looking for Mr. Biden, who is mourning the death of his son, Beau, to enter the race for the past two months. So is CNN, which has a sixth podium waiting for the vice president should he declare hours before the debate. For supporters, Biden is viewed as a no-fail alternative, more likable than Clinton, and more moderate than Sanders.

But time is ticking. As the New York Times recently wrote, "The danger for Mr. Biden, as his advisers know all too well, is that intrigue can easily turn into fatigue."