It's Joe Biden vs. Elizabeth Warren, round 1

Biden and Warren are, and likely will continue to be, the yin and yang of the battle to be the Democratic standard-bearer in the 2020 election. Biden is establishment, incremental and pragmatic. Warren is outsider, swing-for-the-fences and idealistic. Biden has been the race's front-runner since the beginning. Warren is rapidly stealing the spotlight from Bernie Sanders within the party's progressive wing.

They represent, broadly, the two paths before Democratic voters, who will spend the coming months figuring out which option has the better chance of defeating President Donald Trump. And remarkably, they've never shared a debate stage before.

All of which means we -- and they -- will be in uncharted territory tonight. Which is exciting!

That said, both candidates have already provided very clear hints about what they view to be the other's major weakness, and a close reading of those comments tells us a lot about the ground on which Thursday's debate will be fought between its two most important players.

Let's start with how Warren will attack Biden.

Over this past weekend in New Hampshire, all of the major candidates -- including Biden and Warren -- were in the state for its Democratic Party's annual convention. Which is where, from the stage and with thousands of loyal Democrats cheering her every word, Warren said this:

"There is a lot at stake, and people are scared. But we can't choose a candidate we don't believe in because we're scared."

If you think a) that line was accidental or b) it wasn't aimed directly at Biden's electability argument, there's a very hot video company named Blockbuster I'd like to sell you.

Warren's line struck me as a direct repudiation of Jill Biden's assertion in New Hampshire last month that electability is all that any Democratic should care about.

"Your candidate might be better on, I don't know, health care, than Joe is, but you've got to look at who's going to win this election," Jill Biden said. "And maybe you have to swallow a little bit and say, 'OK, I personally like so and so better,' but your bottom line has to be that we have to beat Trump."

What Warren was saying, in essence, was this: We all want to beat Trump. But you don't have to compromise your beliefs in order to bring about that outcome. Don't vote for someone you don't believe in just because he says that the only thing that matters is beating Trump and he's the best person to do that.

But Biden has prepared a counterargument.

"I expect you'll see Biden echo an important point he made during last week's climate forum: We need more than plans, we need a president who can deliver progress on the most pressing issues facing Americans -- which Joe Biden has proven he can throughout his career," a Biden adviser told CNN earlier this week.

Plans are not enough is, again, a very purposeful shot at Warren -- even though her name wasn't invoked by the adviser -- who has premised her entire candidacy on the idea that she has a detailed plan for anything and everything.

Biden's team sees the contrast between his years of fighting and winning political battles and Warren's years spent in academia and her relative lack of legislative accomplishments during her seven years in the Senate.

One of these two candidates has done things and one has talked about what she would do, goes the Biden argument.

Now, it's possible that Biden and Warren will decide that tonight is not the right time to air these attacks in front of millions of people watching on TV. Or that a moment never presents itself where the hits feel like they make sense.

But make no mistake: These are the two defining contrasts of the Democratic primary fight -- unless and until some other candidate (Kamala Harris and Pete Buttigieg being the most likely options) shows that they belong in the top tier.

Doing vs. talking. Voting from fear vs. voting for hope. Insider vs. outsider. A return to normal vs. large structural change. This is the fulcrum on which the fight for the hearts and minds of Democratic voters will pivot. And that battle likely will begin in earnest tonight.