Joe Biden’s prospects for winning the Democratic nomination have waned after both the former vice president’s disappointing fourth and fifth place finishes in Iowa and New Hampshire and Sanders’s strong first-place finish in Nevada. But despite those setbacks, Biden still often outperforms other Democratic contenders against Trump when people are asked about potential general election match-ups.

But how much should we really trust these polls? And is there a deeper explanation for why Biden does well? In other words, is there something unique to his support?

For the last 12 years, I’ve overseen a long-running panel survey of Americans’ political attitudes that sheds light on how their political views have evolved, and one of the questions I’ve consistently asked is who panelists would back in a general election. And as of our most recent survey in late January, they backed Biden over Trump 48 to 40, while backing Warren, Sanders, and Buttigieg by smaller margins of 1 to 3 percentage points over the president. What’s more, 8 percent said they would back Biden but not Sanders, while 3 percent said the reverse.

Now, some of that is probably related to one downside of our survey — we don’t have any respondents under the age of 30. (Respondents had to be at least 18 to participate when the panel began in 2007.) That means some of the voters who are the most pro-Sanders are underrepresented and some of Biden’s supporters are probably overrepresented. (The survey respondents are also likely to be especially politically engaged, seeing as they were willing to take repeated political polls for over a decade.)

Asking respondents about their general election preferences this far in advance isn’t totally meaningless, though. Using our panel data, we found that the candidate respondents told us they’d back during the primaries has often matched whom they backed later in the general election. Consider 2008: Thirteen percent who answered our survey during the later half of the primaries said they would back Hillary Clinton but not Barack Obama in the general. Many meant it: When we asked again about who they supported in the fall of 2008, 33.5 percent of those respondents said they backed Sen. John McCain and another 28.2 percent backed neither major-party candidate.

Likewise, in January 2016, those who said they would back Sen. Marco Rubio but not Trump in the general (11 percent) remained wary of Trump. In October 2016, 25.5 percent told us they now backed Clinton while another 48.2 percent said they remained uncommitted, wouldn’t vote, or backed a third-party candidate.

So simply put, there’s reason to believe that when 8 percent of panelists tell us they’ll only back Biden and not Sanders — or 3 percent, in the case of voters who said they’d only back Sanders and not Biden — many of them mean it, and their minds won’t necessarily change months later.

That raises the question: Just who are these voters who back Biden but not Sanders, his chief Democratic rival? One thing we know is that these voters are more likely to be Republican and conservative and less likely to have supported Democrats in past elections than those who would vote for multiple Democrats. That said, this isn’t just true of Biden-but-not-Sanders supporters; the respondents who said they’d back Sanders but not Biden also tended to be more Republican and conservative.

Those who would back Biden versus Trump — but would not back Sanders — also tend to be older, with an average age of 53 for the Biden backers versus 45 for the Sanders backers. Those who back Biden were also more likely to have a higher income, with a mean income in 2016 of $79,000, as opposed to $60,000 for Sanders’s backers. (However, the sample sizes in each group were too small to be sure that the differences were statistically significant once we account for multiple comparisons.)

Biden also did better than Sanders among respondents in almost every partisan category in our sample, from strong Democrats to strong Republicans. One of the groups among which he did better than Sanders, “weak” Republicans, could be particularly consequential, given that converting some of the other side’s backers is often key to winning against an incumbent.

These results suggest that, as the Democrats decide on who their nominee will be, they are also to some extent deciding on who the election’s swing voters will be. Swing voters do still exist, but they come in different types — and different candidates might alienate different voters. For instance, given what we know about who supports Biden and not Sanders, Biden might be able to win over wealthier, slightly more Republican voters in a general election, while Sanders might be able to build up his margins among the young if he wins the nomination. To some degree, the demographic divisions we currently see in the Democratic primary already foreshadow which coalitions either candidate might build in the general election.