Sen. Elizabeth Warren (D-Mass.) (Andrew Harnik/AP)

The second you try to make a list of, well, anything, you open yourself up to people saying “How could you leave [fill in the blank] off your little list?!" It's sort of the point of lists; they are meant as the starting point of an argument, not the conclusion of one. It's basically why the Internet was invented.

It's no surprise then that my first attempt to handicap the 2020 Democratic presidential field would be met with lots of, um, unhappiness about who I left off the list. Before I get to who I didn't mention, let's go through who I did:

* New Jersey Sen. Cory Booker

* New York Sen. Kirsten Gillibrand

* California Sen.-elect Kamala Harris

* Colorado Gov. John Hickenlooper

* Minnesota Sen. Amy Klobuchar

* First lady Michelle Obama

The biggest oversight from that list — according to the Twitter-verse and my email inbox — is Massachusetts Sen. Elizabeth Warren, the liberal hero who many urged to run against Hillary Clinton in 2016 to no avail. In truth, Warren probably should have been on the list; she's a national figure with a demonstrated fundraising base who emerged in the 2016 election as one of the most blunt critics of President-elect Donald Trump. I left her off because of her resistance to running this time around and the fact that she would be 71 years old on Election Day 2020.

Of course — as has been pointed out to me MANY times over the last 24 hours — refusing to run against Clinton isn't the same thing as refusing to run for an open nomination. And, at 71, Warren would still be three years younger than President Trump in 2020. Fair enough.

Here's a look at some of the other names people mentioned as 2020 contenders and why I agree (or disagree):

* Ohio Sen. Sherrod Brown: The Ohio senator was on the Clinton VP long list but fell off before the field winnowed itself. Brown's blue-collar roots and gruff — in an appealing way — manner make him someone who might be the right fit to bring back disaffected white voters to the party in 2020. Oh, and Brown happens to be from an absolutely critical swing state. One hurdle: Brown is up for a third term in 2018 and could face a real fight for reelection.

* Montana Gov. Steve Bullock: If Democrats want to try to further their gains in the West, Bullock could be an appealing choice. He won reelection earlier this month with a (bare) majority of the vote even as Trump was cruising to victory in the Last Best Place. One major problem for Bullock: He's never raised anywhere near the sort of money he would need to be competitive with some of the bigger names on this list.

* Housing and Urban Development Secretary Julián Castro: An early front-runner to be Clinton's VP, Castro was quickly eliminated. As a young, charismatic Hispanic politician from Texas there's plenty of upside for Castro in the future. But, what does he do between now and 2018 to stay in the national conversation? Castro will be out as secretary of housing and urban development in a month, and there's no other obvious race for him to make to keep his name in the mix.

* Virginia Sen. Tim Kaine: As the party's most recent vice presidential nominee — not to mention a senator from a swing state — Kaine would likely be a top tier candidate if he ran. But, he seems to have already closed that door, telling the Associated Press he absolutely would not run in 2020. Things change, of course, and four years is a long time from now. Kaine has more near-term worries anyway; he's up for a second term in 2018.

* Virginia Gov. Terry McAuliffe: McAuliffe will be term limited out of office in 2017 — Virginia has a remarkably dumb one-term limit law — and will likely be looking for his next move. He is ambitious and optimistic in equal measure, and history suggests he knows more than almost anyone in Democratic politics about how to raise lots and lots of money. But, do Democratic voters really want Bill Clinton's best friend as their 2020 nominee?

* Missouri Sen. Claire McCaskill: McCaskill was one of the earliest endorsers of Barack Obama in 2008 and Clinton in 2016. And as a second term senator from the red state of Missouri, she's clearly got political chops. But, of everyone on this list, McCaskill is likely to have the toughest reelection race in 2018, and it's hard to see how she quickly organizes for a 2020 presidential bid even if she happens to win a third term.

* Connecticut Sen. Chris Murphy: Murphy has seen his profile rise in the Senate thanks to his outspokenness on gun control. And, if Warren didn't run, he could make a case as the most principled progressive in the field. But, Murphy may still be too low profile to make a real run at the nomination and may instead bide his time to move up the leadership ladder in the Senate.

* California Lt. Gov. Gavin Newsom: Follow me here: Newsom gets elected governor of California in 2018 and quickly pivots to a presidential run. Possible? Yes. Probable? No. And having just turned 49, Newsom can afford to wait a while for a better opportunity to present itself.

* Deval Patrick: The former Massachusetts governor — he left office in early 2015 — has long been rumored as a potential national candidate. Patrick ruled out a 2016 bid in late 2014 but didn't totally close the door on running for president down the line. Patrick's profile — two-term governor of a Democratic state, early 60s, African American — could appeal to many looking for the second coming of Barack Obama. It's not clear whether Patrick has genuine interest in the race, however.