(CNN) With 78 days until the Iowa caucuses, the 2020 election will be here before you know it. Every Sunday, I deliver to your inbox 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.

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5. Impeachment, impeachment, impeachment: The steady drumbeat of the 2020 campaign isn't coming from Iowa or New Hampshire. It's coming from Washington and, specifically, Capitol Hill, where the second week of public impeachment hearings is packed with potential problems for President Donald Trump.

There are five hearings featuring eight total witnesses scheduled for Tuesday, Wednesday and Thursday -- a massive increase from the three witnesses over two hearings last week.

The two big(gest) ones to watch:

*National Security Council Ukraine expert Alexander Vindman, who was on the July 25 call between Trump and Ukrainian President Volodymyr Zelensky, and has testified behind closed doors that he believed the conversation was inappropriate and immediately alerted his superior to that fact. Vindman will testify publicly Tuesday morning.

*Ambassador to the European Union Gordon Sondland will testify Wednesday morning in what will be the most-watched (and potentially most consequential) of the week's testimony. Sondland has been identified as taking a call with Trump on July 26 to convey that the Ukrainians were ready to announce an investigation into Joe and Hunter Biden. Sondland also revised his original closed-door testimony to note that he had, in early September, told a top Ukrainian official that the hold-up of nearly $400 million in US military aid was directly due to the lack of an announcement of a Biden investigation. (There has been no evidence of wrongdoing by either Biden.)

If this week on the impeachment front is as bad as last week -- and there is every indication it will be -- Trump will be in a world of hurt.

4. Kamala's collapse continues: The slow-motion disaster of California Sen. Kamala Harris' campaign has been hard to watch. The California senator has gone from one of the front-runners for the Democratic nomination this summer to being forced to run a last-ditch, Iowa-only candidacy today. And according to the brand new The slow-motion disaster of California Sen. Kamala Harris' campaign has been hard to watch. The California senator has gone from one of the front-runners for the Democratic nomination this summer to being forced to run a last-ditch, Iowa-only candidacy today. And according to the brand new CNN/Des Moines Register pol l, it isn't working; Harris is at 3% with a bunch of other longer-shot candidates.)

That external turmoil is reportedly mirrored by internal fighting, with fingers being pointed at Harris' campaign manager, according to terrific reporting by Politico's Christopher Cadelago . Cadelago writes in a devastating piece on Harris' campaign:

"As the California senator crisscrosses the country trying to revive her sputtering presidential bid, aides at her fast-shrinking headquarters are deep into the finger-pointing stages. And much of the blame is being placed on campaign manager Juan Rodriguez."

Having seen a lot of presidential candidacies over my time covering this stuff (aka I am old), I can say unequivocally that this is how campaigns end. Money problems, poll problems and, finally, the blame game. Harris has already qualified for this week's debate (much more on that below) and the next debate in December, but I'm skeptical she can hold on for another week with this current trajectory.

3. Trump's health questions: On Saturday, President Trump made an On Saturday, President Trump made an unscheduled and unannounced visit to Walter Reed National Military Medical Center to undergo a "quick exam and labs" that was, according to the White House, part of his annual physical. "Anticipating a very busy 2020, the President is taking advantage of a free weekend here in Washington, DC, to begin portions of his routine annual physical exam at Walter Reed," explained White House press secretary Stephanie Grisham.

Trump echoed that sentiment in a tweet Sunday morning , casting the visit as "phase one of my yearly physical" and pronouncing that "everything very good (great!)."

But given a) the unscheduled nature of the trip b) the fact that his two previous yearly physical exams were on his official calendar and announced to the media and c) the fact that Trump is 73, it seems as though more questions will (and should) be asked about what exactly happened on Saturday.

All of those questions -- why was it an unannounced visit, why did he start his physical less than a year after his last one, etc. -- may well be easily answered to everyone's satisfaction. But given the way Trump obfuscated about his health as a candidate -- Dr. Harold Bornstein, anyone? -- they are questions that should be answered by this White House.

And then there is the fact that Trump -- in the 2016 campaign against Hillary Clinton and again in 2020 targeting potential rival Joe Biden -- has sought to raise questions about his opponents' mental and physical heath. To insufficiently answer questions about his own health would seem to be, in a word, hypocritical.

2. A new Iowa front-runner: It's been clear for weeks now that South Bend, Indiana, Mayor Pete Buttigieg was in the midst of a second major surge in the race. (Buttigieg's first surge this spring took him for nowhere to the top tier.) Now, the It's been clear for weeks now that South Bend, Indiana, Mayor Pete Buttigieg was in the midst of a second major surge in the race. (Buttigieg's first surge this spring took him for nowhere to the top tier.) Now, the new CNN/Des Moines Register poll proves just how much momentum the man they call Mayor Pete has.

Buttigieg has a 9-point lead over Massachusetts Sen. Elizabeth Warren and a 10-point edge over both Biden and Vermont Sen. Bernie Sanders. Which is remarkable given that Buttigieg was entirely unknown in the state as recently as a year ago.

What's more remarkable -- and more worrisome for Buttigieg's rivals -- is the trend line in Iowa for the mayor. Buttigieg is up 16 percentage points since a September CNN/DMR poll while both Warren (down 6 points) and Biden (down 5 points) lost support over that time. (Sanders gained 4 points from September to today.)

And Buttigieg's approval numbers suggest he will be difficult to topple in the remaining months before the Iowa caucuses. Almost three-quarters of respondents (72%) have a favorable view of Mayor Pete while just 16% have an unfavorable one. That's both the highest favorable number and the lowest unfavorable number in the field. Plus, the 42% who said they feel "very" favorably toward Buttigieg is rivaled only by the 36% who say the same of Warren.

1. It's starting to get late in the debates: On Wednesday in Atlanta, 10 Democratic candidates will take the stage at Tyler Perry Studios. For eight of them, it will be their fifth debate of the primary season. For one -- Hawaii Rep. Tulsi Gabbard -- it will be her fourth. (Gabbard did not qualify for the August debate.) And for billionaire Tom Steyer, it will be his second (straight) debate -- after a late entry in the field causes him to miss the first three set-to's.

While more debates remain before the first vote in Iowa, this week's debate feels like the last best chance for the second-tier (and below) candidates to begin building the momentum they need to seriously challenge for the nomination.

Case in point: Minnesota Sen. Amy Klobuchar. She is at 6% in the most recent CNN/Des Moines Register poll of Iowa -- behind the front-running four but above the low single-digit crowd. Klobuchar has been solid in the debates to date but has struggled to build momentum coming off of her above-average performances. She's running out of time -- and needs to find a way to distinguish herself (and turn that into votes) in this debate.

No matter what happens, Klobuchar will have another debate in December. (She is one of six candidates who have already qualified for the Los Angeles debate.) Which makes the stakes for her high but not as high as the four candidates on stage Wednesday who haven't qualified for next month's debate yet : New Jersey Sen. Cory Booker and technologist Andrew Yang as well as Steyer and Gabbard.

Booker may be in the most tenuous place of those four, as the New Jersey Democrat entered the 2020 race with high expectations that he simply has not met. Unless Booker has a standout performance Wednesday, he may well miss the next debate. And if someone of his stature is missing a debate two months before thew actual vote, it's hard to justify staying in the race.