The top 12 Democratic presidential candidates will take the stage for their fourth debate Tuesday night. WGBH News Political Reporter Adam Reilly gave WGBH Radio’s Arun Rath a preview of what to expect. This transcript has been edited for clarity.

Arun Rath: So the dynamics of this race have shifted since the last debate. Talk about the big change.

Adam Reilly: One of the big changes that I think is fascinating is Elizabeth Warren's steady rise to, I would say, either the front runner position or clearly one of the top two candidates, along with Joe Biden. Back when they debated in September, she was not in that same position, but right now she's opened up a lead in Iowa, a lead in New Hampshire, and she's running second when it comes to national polls, which don't really matter that much since we don't vote nationally. She's running second in the country to Joe Biden.

And when you look at the trend line for those national polls, she has had this steady rise — whereas he, at some points, looked like no one was going to be able to catch him. He's gone up and down and up and down and up and down, with the end result being that he's headed more downwards than upward. So she's in a very good spot. And I assume we will continue to see and hear more of what we've seen and heard from her in previous debates.

Read more: Everything You Need To Know About Tuesday's Democratic Debate

Rath: Slow and steady improvement for Warren and this sort of three-way front runner dynamic with Sanders, Warren and Biden. And we should mention the other big development that, obviously, Bernie Sanders is coming off of recovering from a heart attack. Does this mean that we might see Sanders going after Warren?

Reilly: I think we might see him gently try to highlight some differences between him and her. Now for the record, I've done a couple of these previews, and I think in each of them I've predicted, 'Oh, tonight's the night that we're going to see Bernie Sanders and Elizabeth Warren really go at it.' And I've been wrong in every case. So take what I say now with a grain of salt.

But over the weekend, Sanders made a point in an interview with ABC News. He said that Elizabeth Warren is terrific. She's a good friend, a really good senator, but as he put it, 'She has said she is a capitalist to her bones. I'm not.' So I wouldn't be surprised if he tries to bring that distinction to the forefront.

By the way, one other big change which I haven't even gotten to, is that we now have impeachment proceedings underway, largely because of President Trump's attempts seemingly to obtain Ukrainian assistance in going after Joe Biden's son. And Joe Biden has since then come out and said: I'm in favor of impeachment. Again, the process is going in a way it was not, and it'll be interesting to see if the candidates talk about impeachment and its political ramifications and the cultural ramifications of where we're at right now, because in past debates it's almost been like they didn't want to get sucked down into the impeachment vortex. They were insistent on putting out their visions for the country. But it's this huge thing going on, sucking all the oxygen out of the room when it comes to the national civic conversation, and I don't see how they can avoid it tonight the way they have in past debates.

Rath: It's a strange and obviously completely unprecedented thing that we have one of the candidates who's running as a central player in the impeachment drama of the president. With these things that are coming out about Joe Biden and his son, is that good or bad for Biden in this race?

Reilly; I had thought that it might be something that gives Democrats who maybe like the idea of Joe Biden — but don't think he's quite what he used to be — maybe reason to opt for another candidate, whether it's Elizabeth Warren, Bernie Sanders, or someone else. I don't know that there is quantitative evidence that that has yet occurred, but throughout this campaign, I think we've seen times when Biden seems to get in his own way or not to respond to attacks from Trump or Trump surrogates as effectively as he could. Now just recently we had Hunter Biden give a televised interview in which he said of his involvement with this Ukrainian gas company Burisma: "Was it poor judgment to be in the middle of something that is ... a swamp in many ways? Yeah."

So there he is accepting President Trump's terminology. I think that quote, coupled maybe with some of the struggles that Biden has had over the past few months, might give some Democrats who are voting pause. I'm not sure that we're going to see any of the Democrats on stage echo President Trump's attacks on the Bidens and go after him in a comparable way tonight. That seems to me like it would be taking a big, big risk.