The Democratic field for president is filling up, with each candidate saying they're the one who is most electable. But just what is electability, anyway? That's a question taken up by WGBH News contributor David Bernstein. He spoke with WGBH All Things Considered host Barbara Howard. The following transcript has been edited for clarity.

Barbara Howard: So you have a column out that talks about how each Democrat voter brings to the table his or her own version of who is most likely to be electable, who might beat President Trump. How does that play out?

David Bernstein: Right. For instance, four years ago, people who were very eager about Bernie Sanders also seemed to be convinced that Bernie Sanders would beat the Republican, and still believe today that he would have won because of flaws that Hillary had. And certainly Hillary Clinton backers thought the opposite. They thought that he was unelectable in the general election. So it tends to be self-reinforcing a lot of times. That's really what's happening now — a lot of people who are backers of Elizabeth Warren are the same people who think that the Democrats need a real pugilistic, populist liberal, to go after Trump. If you're favoring someone like Amy Klobuchar, you're going to say that the Democrats need someone from the upper Midwest to win back states like Michigan and Pennsylvania and Wisconsin that the Democrats lost last time. So which comes first — the belief in what will beat Donald Trump, or liking the candidate? It's hard to say which came first.

Howard: Is there a danger that by bringing what an individual perceives as the most electability can actually undermine the ultimate candidate? In other words, when Bernie Sanders didn't make it and Hillary Clinton did, it took a long time to get his people to rally around her. Is that part of this?

Bernstein: That definitely can play out, because it's going to be part of the conversation as these candidates do battle, and as the backers do battle over the next year, plus. And that's going to leave some tough feelings on both sides, both because of what they liked about the particular candidate, but also because if you really believe that you need a candidate like Klobuchar and someone else gets nominated, you're going to think that they made a bad mistake and may have cost the presidency. So it does make it tougher, but also, the opinions may change as the facts on the ground change about how beatable Donald Trump is anyway. So electability may not be as important by the end of this process as it seems now.

Howard: We are early in the process. How soon can we really begin to assess electability?

Bernstein: Well this is really a key time, usually. Usually what's happening at this point, a year ahead of the first primary and first caucus, is you have the insiders behind the scenes who are really weeding out the unelectables by not writing big checks, not putting unions or other endorsers behind those candidates. That's how this process of whittling often plays out at the beginning. It's really based a lot on electability. The most electable are the ones who connect with that kind of insider support, and then it goes to the voters later on in the primary, to choose among electable candidates. This time around, it seems like voters and activists are getting involved very early and are going to be part of that weeding process.

Howard: We kind of saw that in the Congress shift, because we've seen a lot of people who came out of nowhere without big money behind them who are in Congress now.

Bernstein: That's right. It's been shown now that, for instance, you can raise a lot of money as a relative outsider candidate for Congress, or like Bernie Sanders for president. You can raise money nationally online in $25, $50 chunks, and raise enough to be competitive in your race. Some candidates who the insiders might think are unelectable, they don't want to back those people. But they’re still going to be alive because they can convince the grassroots and the rank and file Democrats to support them.

Howard: We've been talking about the Democrats challenging the presumptive nominee of the Republicans. That of course would be Trump. But is there a chance that a Republican will challenge Trump successfully?

Bernstein: Well I think it's a big longshot at this point, that there will be a successful challenge. But if things happen with Donald Trump — if there are indictments, if there's a bad report from the Mueller investigation, or if the economy goes bad, then they'll be ready to try to take advantage. Then of course it changes what the Democrats want. You know, if it looks like Donald Trump is in trouble for even getting re-nominated, then that changes what voters on the Democratic side think is electable when they're going to their primaries. I think this battle over the definition of electability is going to play out in very strong ways over the next few months, and we'll see whether it gets settled at that point or whether that's going to be a battle all the way through the primaries.

Howard: That's WGBH News contributor David Bernstein.