The first of six debates among those vying for the Democratic presidential nomination takes place tonight in Las Vegas, a fitting backdrop for such a high-stakes event.

CNN is the host broadcaster and Anderson Cooper is moderating the two-hour event beginning at 8:30 p.m. ET.

Hillary Clinton is the frontrunner heading into the debate, but Senator Bernie Sanders from Vermont has been closing in on her in some key primary states. The pressure on them, and the other candidates, to perform well tonight is high. Here are a few things to watch for:

1. Clinton vs. Sanders

The CNN ads for tonight's event are typical of the cliché that debates are akin to candidates squaring off in a boxing ring. The ads feature Clinton vs. Sanders, barely noting there will be three others on stage. The dynamic between the two leading candidates, facing off in a debate for the first time, will be something to watch.

Sanders boasts that he's never run a negative ad in all of his years in politics and sticks to positive campaigning. So we likely won't see Sanders make any comments about Clinton's face (like Republican hopeful Donald Trump did about his rival Carly Fiorina), or her hair for that matter, but will he attack her record?

Will he be tough on her? Accuse her of flip-flopping on the Trans-Pacific Partnership (TPP) trade deal? Denounce her judgment in using a personal email account while secretary of state and causing a scandal? Sanders might be more inclined to let the other candidates do that for him, but maybe not.

Whether Clinton goes after Sanders or leaves him alone will also be something to watch.

She's said little about him in the campaign so far, preferring to state what she's for and against and not talk about his positions. During the debate she will need to emphasize their differences and argue why her ideas are better, but without looking like she's going too hard on him. She doesn't want to turn off his supporters because she needs to appeal to the base of support that he's been building. She won't be able to do that if she comes off like a conceited know-it-all.

2. The other candidates

Yes, there are other people running besides Clinton and Sanders, but they are struggling for attention.

Martin O'Malley, the former governor of Maryland, is among the candidates struggling for national attention in the Democratic presidential nomination race. (Charlie Riedel/Associated Press) That's why the stakes are especially high tonight for Martin O'Malley, Lincoln Chafee and Jim Webb. They get a prime-time, national television audience and they have to make the most of it. This is their chance to introduce themselves to voters across the country, and the pressure is on to avoid gaffes and to stand out from their competitors.

O'Malley is the former governor of Maryland, Webb is a former Virginia senator and Chafee is a former senator and governor in Rhode Island. These are experienced politicians who should be confident on a debate stage. Watch for whether they take advantage of this opportunity to remind the country that they are in the race.

3. Running against Obama

It's easy for Republicans to criticize President Barack Obama and his policies, but Democrats have to be more delicate in how they go about it. They want to keep the White House in their party's hands, of course, so they don't want to bash him too badly. However, they need to tell Americans what they would do differently and why.

Watch for how they distance themselves from the president. Clinton is in an especially awkward position to do this because she was part of Obama's administration as secretary of state. She's already set herself apart from him on some issues, the recently negotiated TPP being a big one. Watch how she does that during the debate, and whether she moves farther away on more issues.

4. Will Joe Biden show up?

Vice-President Joe Biden is considering joining the race for the Democratic presidential nomination. (Andrew Harnik/Associated Press) The vice-president is mulling whether to join the race and he is nearing a decision. Some pundits say his heart wants him to go for it, but his head may be telling him otherwise, particularly because he is grieving the death of his son Beau from cancer a few months ago. If Biden does decide to challenge Clinton, he's not likely to announce it by walking out on stage tonight at the debate, although CNN has said they would let him join the festivities.

Even if Biden doesn't physically show up, his presence could still be felt. Some say Biden might be motivated to seek the nomination because Clinton isn't doing as well as everyone expected. The popular vice-president could act like a saviour for the Democrats by trying to ensure the White House stays in their hands. Clinton has to shine tonight to prove to Democratic voters that they don't need Biden to come to the rescue.