(CNN) Welcome to 2020! With just 29 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.

In each episode of his weekly YouTube show, Chris Cillizza will delve a little deeper into the surreal world of politics. Click to subscribe!

If you zoned out of politics over the break, you likely missed the news that former San Antonio Mayor Julián Castro dropped his presidential bid

That decision culled the Democratic field to 14. Yes, 14 people are STILL running for the Democratic presidential nomination, despite the fact that less than half that number can make an even marginally credible case that a path exists for them to be the nominee.

And we are not six months out from a vote! We are 29 days away from the Iowa caucuses! Which means that if you are still polling in the low single-digits, well, it may not happen for you this time around.

Why does the size of the field matter? Well, take Iowa. If any candidate's support in each individual caucus site doesn't reach 15%, then that candidate is declared not viable and his or her supporters are asked whether they want to caucus instead with a viable candidate's group.

How those decisions get made -- and whether any of the candidates not expected to make the viability threshold indicate who they would want their supporters to back if they don't make it -- could be the difference between winning and losing for the top tier.

Once we are past Iowa, how the bottom tier of the field reacts also matters. Polling suggests New Hampshire -- primary date: February 11 -- is extremely close, and an endorsement of a top-tier candidate by a candidate leaving the race (even if it's 1% of support that moves) could really make a difference.

4. There's a debate coming ... : Suffering from debate withdrawal? Never fear! A week from Tuesday Suffering from debate withdrawal? Never fear! A week from Tuesday the seventh debate of the Democratic primary season is set to take place in Iowa. (And it's hosted by CNN!)

This is not only a) the first debate of 2020 and b) happening just two weeks before the Iowa caucuses but also c) the smallest field yet.

As of today, just five candidates have made the January debate stage: Joe Biden, Bernie Sanders, Pete Buttigieg, Elizabeth Warren and Amy Klobuchar.

That's because the Democratic National Committee has raised the qualifications; candidates now need at least 225,000 unique donors nationwide and a) 5% in four party-approve polls approved by the party between November 14 and January 10, or b) 7% in two early-state polls.

Businessman Andrew Yang, who has met the donor threshold but has only one poll showing him at 5% or more nationally, has called on the Democratic National Committee to pay for a series of polls prior to the January 10 cutoff to make the next debate. The DNC has refused.

Making that January 14 debate stage is a very big deal. It's the last debate before the Iowa caucuses, a last chance -- in a very high-profile setting -- for candidates to make their pitches to the widest possible audience.

3. Remember impeachment?: Sometime in the near future, the Senate is likely to begin the process of conducting an impeachment trial to debate and decide whether President Donald Trump should be removed from office.

Which is a VERY big deal. (Obviously.) But because of the extended Christmas break -- Trump has been in Florida for the last two weeks -- and then the drone strike attack that led to the death of Iranian commander Qasem Soleimani last week (much more on that below), impeachment seems so, well, 2019.

Just a quick refresher, then: We are still in a bit of a stand-off between House Speaker Nancy Pelosi (a California Democrat), who has so far refused to name House impeachment managers until she gets a better sense of how the Senate trial will run, and Senate Majority Leader Mitch McConnell (a Kentucky Republican), who has shown zero interest in letting Pelosi influence his plans.

"For now we're content to continue the ordinary business of the Senate," McConnell said Friday of scheduling an impeachment trial.

Despite all of the posturing, there almost certainly will be a Senate trial that begins (and maybe ends) this month. Which poses all sorts of interesting challenges for the likes of Sens. Bernie Sanders (Vermont), Elizabeth Warren (Massachusetts), Amy Klobuchar (Minnesota) and Cory Booker (New Jersey).

All four need a strong showing in the Iowa caucuses on February 3. And history suggests that planting yourself in Iowa for the final month before the caucuses is the best way to make that happen. But if an impeachment trial -- only the third in history! -- is happening in DC, it may be tough for these senators to walk away from their official duties at such a critical time.

Who does that advantage? Former Vice President Joe Biden and former South Bend, Indiana, Mayor Pete Buttigieg -- neither of whom have jobs that will bog them down in DC.

2. $$$$ (and what we still don't know): Here's what we do know about the Here's what we do know about the fundraising race in the Democratic primary : Sanders ($34.5 million raised between October 1 and December 31) continues to wow followed by Buttigieg ($24.7 million.) Biden had a bounce-back quarter at $22.7 million raised while Warren slowed slightly with $21.2 million collected. Yang brought in $16.5 million.

We can draw a few conclusions from those numbers. Sanders' online, small-dollar network continues going strong. Buttigieg is no fundraising fluke. Biden had a very good past three months, and his fundraising reflects it. Warren struggled somewhat after a bonkers summer (in fundraising and polls). And Yang's number may be the most impressive of all -- given that he started running for president with almost zero fundraising list or base.

The big money question we still don't know is how much cash all these top-tier candidates have left to spend in the new year. (Campaigns aren't required to detail all of their fundraising numbers until they file their report with the Federal Election Commission on January 31).

And the answer to that question may well be more important now than how much each of the candidates raised over the last three months. Why? Because we are entering a phase of massive required spending across a huge playing field. A month after the Iowa caucuses on February 3(!), you have "Super Tuesday," when 14 states -- including monsters like California and Texas -- are set to vote.

Running competitive campaigns across that sort of national playing field will require millions. So who will have that sort of money in the bank?

1. The Iran question: The drone strike, authorized by Trump, that led to the The drone strike, authorized by Trump, that led to the death of high-ranking Iranian commander Qasem Soleimani pushes foreign policy -- and national security -- to the forefront of the 2020 race.

While it's not yet clear whether we'll be talking about it -- or its reverberations -- in November, there's no doubt that Trump's decision and its impact on the broader Middle East will be a major topic of conversation in the run-up to the Iowa caucuses early next month.

And already you've seen at least one 2020 Democrat struggling to find the appropriate response.

Warren initially released a statement calling Soleimani a "murderer" while also raising questions about Trump's strategic goals and the possible dangers created in the region.

Amid an outcry from the left, Warren shifted within the first 24 hours of the strike, describing it as an "assassination" and leaning much more heavily into the critique of Trump. (Sanders described the killing as an "assassination" immediately.) And Warren went even further in an interview Sunday on "Meet the Press," saying, "Donald Trump is very upset about this upcoming impeachment trial. But look what he's doing now. He is taking us to the edge of war."

Biden has been more measured in his response, calling the killing of Soleimani a potential fire-starter in a region ready to explode.

The question: What do early state Democrats want out of their nominee on these issues? Biden's long experience and steady hand? Or the more non-interventionist views of Sanders?