The Democratic field’s primary beneficiary of that priority has, so far, been former vice president Joe Biden. Voters consistently see him as the most electable candidate and, probably as a function of that, support him most strongly in the crowded field.

A new Quinnipiac University poll shows that discrepancy at work. Biden has the support of about a third of Democrats, but just over half think he has the best chance against Trump. The other five leading candidates all have more people expressing support than saying that they think they pose the biggest risk to Trump. (That’s visualized below by the diagonal line. A dot on the diagonal line would represent equal support and views of electability. A dot above the line indicates a candidate with more support than belief that they can win.)

This is, admittedly, a not-very-helpful way of asking the question. Polling from YouGov conducted for HuffPost has asked how electable each candidate might be, showing that, in June, Biden and Sens. Elizabeth Warren (D-Mass.), Kamala D. Harris (D-Calif.) and Bernie Sanders (I-Vt.) are all seen as about equally likely to beat Trump. But when Quinnipiac asks who’s most likely? Biden, by a wide margin.

That’s useful in part because it shows us the relationship between how people view electability and whom they choose to support. For example, look at what happens when we add to the graph above lines showing how the leading Democrats were seen in the last two polls Quinnipiac released that included an electability question.

See what happened there? Let’s put a fine point on it, focusing on Harris and Biden.

In the July 2 poll, taken shortly after Harris successfully fitted Biden with cement shoes during the first Democratic debate, her support surged relative to the late-April poll. In the most recent survey, with that debate fading into history, Biden’s lead relative to Harris’s rebounded. Moving in tandem with overall support? Perceptions of electability.

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The movement in that regard from April to July 2 makes sense. Harris exposed Biden to a very effective attack in the debate, the sort of thing that can have a direct effect on the outcome of an election. Given that, it makes sense that people would see Biden as less electable and Harris as more electable relative to April.

But why is Biden now rebounding? What’s happened to make him seem more electable?

Well, one thing that has happened is that there have been a number of polls showing Biden beating Trump nationally by wide margins. The Washington Post and its partners at ABC News released one earlier this month. NBC News and the Wall Street Journal dropped one in the middle of the month, and Fox News released one (that made Trump mad) late last week. Each showed Biden with about the same lead.

Harris didn’t fare as well, ending up in a tie with Trump in all three polls. So, was that it?

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Maybe. The Quinnipiac University poll was in the field (that is, being conducted) while the Fox News poll came out. Maybe that news bolstered perceptions that Biden could win.

Or maybe it goes the other way. Maybe Harris’s gain after that first debate faded and Biden was the beneficiary. In RealClearPolitics’ average of polls, Biden’s about a point lower now than he was at the time of the debate. This could be a reversion to where Biden was before his poor debate performance — and a corresponding reversion of the sense that he’s also the most electable.

In other words, maybe support drives the perception that Biden will win. We would have a better sense of how the two factors interplay if Biden suddenly slipped from the lead in the Democratic field: Is the fact that he’s leading the Democrats spurring some who don’t want him to be the nominee to nevertheless think he’s got the best shot against Trump? Remember, he was the only major candidate who was below that diagonal line above, meaning that he had more people thinking he was the best bet against Trump than wanted him to be the person running against the president. If he suddenly drops to second place, do those perceptions change?

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In May, we evaluated past primary polling to reveal another critical point: People aren’t necessarily great at evaluating electability. In late 2007, presidential candidate Barack Obama was viewed as much less electable than Hillary Clinton. And then guess what happened.