WASHINGTON — The first Democratic presidential debates, Wednesday and Thursday nights, have been anticipated as the most significant moments of the race so far, as 20 candidates try to distinguish themselves on a national stage.

The event will test the early front-runner, Joe Biden; will give his closest rivals opportunities to rise; and could make or break candidates at the back of the pack. A viral moment might propel a candidate or create a lasting stain.

Here’s what we will be watching for as the candidates meet in Miami.

Warren Wednesday

Sen. Elizabeth Warren arrives with momentum: A recent Monmouth University poll found her support up to 15 percent, from 10 percent in May, and in other surveys she appears to be closing in on Sen. Bernie Sanders in second place.

>> READ MORE: Here’s what Philly teachers asked Elizabeth Warren on her first 2020 campaign stop in Pa.

Now she has a chance to dominate the first night, since a random drawing made her the only top-five polling candidate appearing Wednesday. But that also might make her the most obvious target, and she won’t get a chance to directly go at rivals such as Biden or Sanders.

If she turns in a strong performance, though, Warren might enhance her claim to being the top alternative to Biden.

Biden on the spot

Biden has a substantial polling lead, but is coming off a shaky week that underscored questions about whether he can keep it. The former vice president’s comments about finding civility with segregationist senators in the 1970s were reminders of Biden’s age and his penchant for gaffes.

He has kept a light media schedule and mostly held tightly staged events, but won’t have that comfort zone Thursday night, when rivals are likely to make the front-runner a target. Biden will have to show he can argue on his feet, avoid mistakes, and joust in real time with younger competitors eager to make a mark.

>> READ MORE: In Philly rally, Joe Biden offers unity to replace Trump’s ‘hatred’

He has little to gain, but avoiding damage would be a win.

Booker’s opportunity

There has been a strange disconnect in Sen. Cory Booker’s campaign. The New Jersey senator has won raves for his speeches, but his support in polls has stalled to around 1 percent to 3 percent.

Booker, though, won widespread attention for his forceful response to Biden’s comments last week, and now enters a forum that could play to his strength: oratory.

And Booker will be one of the bigger names on stage Wednesday, increasing his chances to stand out. It’s an opportunity to gain the traction that has eluded him so far.

>> READ MORE: Cory Booker, in Iowa for his first campaign stop, pitches inspiration over ideology

Policy distinctions

On the issues that most concern Democratic voters — climate change, health care, abortion, guns — the candidates generally want to move in the same direction. But some want to go much farther and faster than others, creating distinctions between self-described pragmatists and those who want to swing big.

>> READ MORE: For some 2020 Democrats, incrementalism is out. Bold liberal ideas are in. They come with risks.

While views will vary, this debate will be the first time for many voters to tune in and see what the party as a whole is all about as it seeks to oust President Donald Trump.

But with speaking time likely limited, it may be difficult for candidates to draw clear distinctions on details. Those who hope to do so will have to be ready with sharp, and quick, arguments.

The candidates also seem likely to face questions about impeachment, a potentially tricky issue when much of the Democratic base supports the move but most of the country opposes it.

Sanders vs. Biden

Sanders thrived in 2016 when he drew a one-to-one contrast with Hillary Clinton.

Now, though, there’s a sprawling contest with virtually every flavor of Democrat available, and while Sanders has polled near the top of the field, he no longer has the benefit of being the one alternative.

Still, he’ll be standing next to Biden at center stage Thursday night, and already has assailed the former vice president for supporting the Iraq War and trade deals. As in 2016, the debate gives Sanders a chance to contrast himself with the choice of more moderate Democrats, and perhaps spark his campaign.

>> READ MORE: Bernie Sanders, making pitch to Trump Country, points to Erie, Pa.

The wild card

With 20 voices spread over two nights, the format itself is a wild card.

Each night has a different mix of political profiles, policy views, and polling stature.

Given that speaking time will be at a premium, do candidates work to introduce themselves? Stick to scripts? Swing at rivals? Or hunt for a viral moment that can carry on beyond this week?

They may not have enough time to do it all.

Buttigieg and Harris

Mayor Pete Buttigieg and Sen. Kamala Harris will flank Biden and Sanders on Thursday night, and each has a chance to offer something different from the 70-something men in the lead.

Harris’ prosecutorial skills have helped her stand out at Senate hearings, and if she can create another moment it could jump-start a campaign that has slowed after a fast start.

Buttigieg, meanwhile, has come from seemingly nowhere (OK, he comes from South Bend, Ind.) to force himself into the national discussion. But he’s facing a crisis after a white South Bend police officer fatally shot a black man June 16.

Standing next to Biden, Buttigieg might have a chance to put a fine point on the generational change he has emphasized so far.

>> READ MORE: In Iowa, Pete Buttigieg says his presidential campaign is here for more than a ‘moment’

Make or break at the back

With 24 candidates in the running (including four who didn’t meet the polling and fund-raising criteria to qualify for this debate), many are already anonymous and at risk of staying that way. This might be one of the last chances for people in the back of the pack to show that their campaigns are viable.

There’s another debate in July, and after that the qualifying criteria get tougher, so some of these candidates could effectively get pushed off the stage if they don’t build some support fast.