(CNN) With 57 days until the Iowa caucuses, the 2020 election will be here before you know it. Every Sunday, I outline the 5 BIG storylines you need to know to understand the upcoming week on the campaign trail. And they're ranked -- so the No. 1 story is the most important of the coming week.

5. Mayor Pete under the microscope: It appears as though the rest of the Democratic field has -- finally -- woken up to the reality that if the Iowa caucuses and New Hampshire primaries were held tomorrow, South Bend Mayor Pete Buttigieg could very well win both.

While those facts have been clear for at least the last month, Buttigieg had been largely left alone by his rivals -- up to and including the November debate, in which no one this side of Hawaii Rep. Tulsi Gabbard was willing to offer any sort of sustained critique of the mayor.

No more. Late last week, Massachusetts Sen. Elizabeth Warren called on Buttigieg to make all of his fundraisers open to the press "so that anyone can come in and report on what's being said." And The New York Times editorial board provided Buttigieg's opponents with even more political ammo with an op-ed calling on Buttigieg to speak more openly about his time at McKinsey & Company, an elite consulting firm. (Buttigieg has said he is restricted from talking about his experiences at McKinsey because of a non-disclosure agreement -- and that he has asked to be released from the NDA, to no avail.)

What Buttigieg is about to go through is a rite of passage for any candidate who looks like a potential nominee. You simply don't get through a contested primary fight without your warts being examined. That's especially true for someone with Buttigieg's profile -- a 30-something mayor whose only experience in elected office is as the mayor of his hometown.

How Buttigieg navigates all the incoming he is beginning to face will be a telling indicator of whether he is a flash in the plan or someone who has the real potential to be the party's nominee next fall.

4. The Kamala coalition: It's easy to overestimate what the It's easy to overestimate what the surprise departure of California Sen. Kamala Harris means for the Democratic race. After all, she rarely polled above mid-single digits and hadn't been a major factor in the contest since mid-summer at the latest. (Had she been more of a factor, she wouldn't have dropped out!)

But almost as soon as Harris was out, both New Jersey Sen. Cory Booker and former San Antonio Mayor Julián Castro were seeking to fill the void they perceived the California Democrat left.

While Booker and Castro are undoubtedly earnestly concerned about the reasons behind Harris' decision to leave the race, they are also motivated by politics here. As of today, only six Democrats have qualified for December debate -- and they are all white. (Much more on the debate below!) Booker and Castro are trying to use that lack of diversity to build momentum that has been sorely lacking in their own campaigns for months now.

Will it work? Probably not given that Harris wasn't a top-tier candidate when she dropped out of the race. But for candidates like Booker and Castro, who may be on the verge of being forced from the race, it's worth a shot.

3. Here comes Joe Biden!: Suddenly the former vice president is the hottest candidate in the race -- or at least a hot candidate in the race.

How has Biden done it? Slow and steady-ish. He's never panicked -- even amid pans of his debate performances, poor fundraising and attacks from liberals on Twitter -- and stayed focused on his North Star: The African American vote. And black voters simply haven't wavered in their support of him -- nationally and in the critical early state of South Carolina.

Biden has also been aided by the unrelenting focus on both he and his son, Hunter, by the current resident of 1600 Pennsylvania Avenue. If Biden's strongest argument is that he is best-positioned to beat Donald Trump, then the President's fixation with Biden seems to suggest the incumbent is worried about him.

2. Debate deadline day is coming: As of today As of today , only six candidates will be on the debate stage in Los Angeles on December 19 . That's the smallest debate grouping -- by four candidates! -- we have had in the 2020 race to date.

For the nine candidates who haven't yet qualified for this last debate of 2019, they have until only Thursday to do so. Which means by Thursday, they need more than 200,000 unique donors and either 4% in at least four national or early state polls or 6% in two early state polls.

Only two candidates appear to have a real chance at qualifying before Thursday's deadline: Andrew Yang and Gabbard. Both have already passed the 200,000-donor threshold and need a single poll to make it.

In Yang's case, he has three early state or national polls that have him at 4% or better, meaning he needs just one more. Gabbard also has three early state or national polls that show her at 4% and she has one early state poll that has her at 6%. Meaning that if she gets 4% in a qualifying national poll or 6% in a qualifying early state poll before the Thursday deadline, she's in.

Among the candidates almost certain to be on the outside looking in? Booker, who, if he can't make the debate stage, might very well be done in the race.

1. All impeachment, all the time: Even with the first vote of the 2020 primary season now just two months away, the story in politics -- and the country -- continues to the impeachment of President Donald Trump.

Nothing changes this week as the House Judiciary Committee is set to present its evidence against Trump Monday in its second public hearing, and a vote to approve articles of impeachment could come as soon as later in the week.

The challenge that presents for every 2020 Democrat is obvious: How do you break through the impeachment focus, with voters and the media? While the question is obvious, the answer is not so simple. Every candidate running for the Democratic nomination -- aside from Gabbard -- has long been in favor of impeaching Trump. So there's no new news to break by simply repeating that Trump has to go.

The approach most of the candidates have taken is to largely ignore impeachment on the campaign trail, responding only if asked questions about it -- under the belief that most potential voters are more focused on nominating someone who can beat Trump next November than on getting him out of office this second.

That's probably the right approach. But with everyone's attention diverted to Washington, it's harder to move the needle in Des Moines, Manchester, Reno and Columbia.