Ten Democratic presidential candidates are set to debate in Houston tonight. It is the fist debate to feature the three front-runners — Joe Biden, Bernie Sanders and Elizabeth Warren — all on the same stage. WGBH News' political reporter Adam Reilly discussed the debate with Rupa Shenoy. This transcript has been lightly edited for clarity.

Rupa Shenoy: So we're obviously talking about Warren, being in Massachusetts.

Adam Reilly: She's a big part of the story this evening, absolutely.

Shenoy: And she's been making very big strides.

Reilly: Yeah, she has had this very impressive, methodical rise in the polls, which I think has been driven by a real knack for retail campaigning, which you see if you follow her around, whether it's here or in Iowa. She's very, very good on the stump. And she also has become almost a parody of herself, in a good way, in terms of her ability to issue a plan for seemingly every issue under the sun.

Shenoy: Joe Biden is still the front-runner. It seems like he's come down off this period he had where he was having some trouble and getting attacked. Now that's over, it seems like.

Reilly: I don't know if it's completely over. He hasn't been attacked as much as we might have seen when Kamala Harris went after him on busing, in that very memorable debate moment, for example. But he's also had all these self-inflicted wounds where he gets dates wrong, or names wrong, or creates composite characters out of multiple experiences. I don't get the impression with these mistakes that Biden has made that he is trying to fudge things in a way that's beneficial for him. It more just seems like he's not really sure what the actual thing that happened is.

I think the net effect has been negative, and it's made, I would argue, a lot of Democrats wonder if he is the guy they want to be the nominee who's going to go up against President Trump. A big thing I'll be watching for from him tonight is whether he hurts himself in any of these ways he doesn't have to, whether he shows his age. He is not a young candidate. Neither is Bernie Sanders, neither is Elizabeth Warren, but Biden I think is showing his age — possibly because so many of us are familiar going back decades — showing his age in a way that the others have not been.

Shenoy: Elizabeth Warren has this quote going around, "I agree with Bernie Sanders." So she comes off as a Bernie Sanders supporter. Is that going to change tonight?

Reilly: She comes off as a Bernie Sanders supporter, and he comes off as an Elizabeth Warren supporter, and so far at least they've managed to not really aggressively go after each other. I don't know if it's going to change tonight, but it has to change at some point, because peeling away Sanders voters from Elizabeth Warren or doing the opposite for Bernie Sanders — that could be the difference winning the New Hampshire primary, maybe going on to become the nominee, and just dropping out of the race at a fairly early date. So at some point I would assume their seeming mutual non-aggression pact has to fall by the wayside.

Shenoy: There's also this possible moment tonight where these lesser candidates or candidates that aren't as far along might take a shot at a viral moment to make a name for themselves.

Reilly: Like that Kamala Harris moment that we mentioned, which I think, looking back on that now, I think it's pretty clear that she had teed this up as something she was going to go after Biden on to make a splash. It worked for her in the short term, doesn't seem to have worked for her in the long term. She's not mentioned as one of the top tier candidates. I would not be at all surprised to see her, Julián Castro, Cory Booker, Pete Buttigieg — who we heard a lot about early on and has seemed to fade into this mild-mannered young man persona of late — I'm sure they would love to replicate what Kamala Harris did in that debate a while back. But I think one thing we've seen is that it's tough to take a moment like that and make it work.

Shenoy: One thing that's been looming over the Democratic field is Obama's legacy. Some of the more left-leaning candidates have been critical. Biden's made this a centerpiece of his campaign. Do you think he's going to stick with that tonight?

Reilly: Oh yeah, he absolutely will. I think he's all in on him having been President Obama's good friend and partner. That is probably the center of his political brand right now, for better or for worse. But on issues like immigration, the Democrats are moving away from President Obama's legacy. And I think that there's a sense in the field also on economic issues, about the lack of accountability for people who helped pave the way for the financial meltdown, the lack of accountability that they saw maybe help pave the way for the election of President Trump. So yeah, Obama's moderate, slightly left of center approach to governance is a little out of step with where the party seems to be going right now.