(CNN) With 190 days until the Iowa caucuses and a make-or-break debate just days away, expect the 2020 candidates to come out swinging this week. Every Sunday, I round up the 5 BIG storylines you need to know to understand the upcoming week on the campaign trail. And they're ranked -- so No. 1 is the most important.

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The second debates are (almost) here! On Tuesday and Wednesday in Detroit, the 20 top candidates for the 2020 Democratic presidential nomination will share a stage.

The stakes are incredibly high given a) how impactful the first debates were, with California Sen. Kamala Harris surging off of a strong performance and b) the fact that the Democratic National Committee's qualifications to make it into the third debates in September are significantly higher and will likely winnow the field significantly.

I'll be in Detroit all week -- CNN is sponsoring these debates! -- but here's a primer of the five candidates with the most to lose with a poor debate performance.

5. Pete Buttigieg: The mayor of South Bend, Indiana, has been the story of the race so far. But his spring surge has faded into a summer status quo. Buttigieg is now a second tier-candidate -- although he's at the front of that pack. His massive fundraising in the second quarter -- $25 million raised -- should help with that problem, but a strong debate performance Tuesday night could also go a long way to restarting the buzz machine for him.

Buttigieg was very, very solid in the first debate; he seemed intent on making sure no one thought he was too young and inexperienced to be president. He played it safe. Which is fine! But Buttigieg is likely to take more incoming in this debate than he did in the first one -- which is an opportunity and a danger.

To date, Mayor Pete has outperformed expectations in every possible regard in the race. Can he do it in this debate, too?

4. Kirsten Gillibrand: The New York senator seems to have all the right traits -- female, liberal, a powerful voice in the #MeToo movement -- to make some noise in the wide Democratic field. It just hasn't happened for her yet, and, man, time is running short.

Gillibrand, barring some sort of moment in this debate, will be in danger of missing the third debate -- and that could be the end. Which means: Now is the time for her to start making her move.

What might be her play? She seemingly gave a hint this past week when she accused nameless "Democratic candidates running for president right now who do not believe necessarily that it's a good idea that women work outside the home." Hmmm ... maybe name names?

3. Bernie Sanders: While former Vice President Joe Biden caught most of the flack for a less-than-stellar performance in the first debate, Sanders wasn't all that much better. And since that first debate, the Vermont senator's poll position has only grown more tenuous -- with Massachusetts Sen. Elizabeth Warren continuing to eat into Sanders' support among the most liberal voters in the party.

Sanders has the most debate experience other than Biden in this field. Which should accrue to his benefit. But, it feels like Sanders has only one gear -- shouty and scoldy -- and, at least in the first debate, that didn't turn out so well. Can Sanders adjust? And is he willing -- and able -- to take on Warren, to try to win back some of the voters she has peeled off of him?

2. Beto O'Rourke: No candidate has underperformed expectations like the former Texas congressman. When he entered the race in March, he was seen as one of a handful of candidates with a real claim on the Democratic nomination. Today, he finds himself -- still -- in search of a message and momentum, hanging solidly in the middle of the second tier.

O'Rourke's performance in the first debate did him no favors. He was too robotic and got beat, badly, in an exchange on immigration policy with former San Antonio Mayor Julián Castro. O'Rourke's team is promising more fireworks this time around, with a particular eye on Buttigieg, who sort of stole O'Rourke's mojo in the race.

O'Rourke simply isn't going to get all that many more chances to live up to his promise in this race. Tuesday night's debate is a huge one.

1. Joe Biden: No one has more to lose (or possibly gain) than the former vice president. Biden was very mediocre in the first debate of the race last month -- seemingly caught entirely flat-footed by Harris' attack on his votes on school busing. It's very unlikely that Biden will be that surprised again by an attack from one of his rivals -- and he'll have a chance to exact a bit of revenge, too, as he shares a stage with Harris on Wednesday night.

But if Biden is bad -- as in, if he looks, again, as though the speed of the game is too much for him, he is going to spend the next month (at least!) fighting off questions about his age (he's 76) and whether the race has passed him by. All of that could cost him his frontrunner status.