Every candidate wants to lead in the polls. It’s not always the most useful thing to be outpacing your competitors; donors, for example, may be less likely to pony up if they think you’re going to win regardless. But it’s obviously better to be winning a political race than not to be winning it. That’s just math.

But there are leads and then there are leads. A lead — non-italicized — is what former vice president Joe Biden has in the 2020 Democratic presidential primary contest. He’s somewhere south of 30 percent in most polls, with Sen. Bernie Sanders (Vt.) somewhere north of 20. Biden is ahead, but not by a ton. A lead, on the other hand, is what Hillary Clinton had four years ago right now.

We created a Twitter account that will show you how past candidates were faring at this point in a cycle. On this day in 2015, Clinton was up by about 57 points.

That’s a lead. But, of course, that’s not where she ended up. Sanders, Clinton’s primary challenger in 2016, closed the bulk of that gap over the course of the contest and gave her a much tougher fight than anyone was expecting. Eight years earlier, in April 2007, Clinton had a narrower lead over Barack Obama — and ended up losing the nomination.

The pattern, though, offers a mixed preview for Biden: Two of the four Democratic candidates who led in an average of polls in April the year before a presidential election (excluding incumbents) went on to win the nomination.

The exception besides Clinton in 2008? Former senator Joe Lieberman (Conn.) in 2004. (We included Biden’s 2008 performance, as well.)

On the Republican side, the results were the same. Twice, the front-runner in April went on to win the nomination — George W. Bush in 2000 and Mitt Romney in 2012. Twice, the leading candidates lost. Those were former New York mayor and now Donald Trump defender Rudolph W. Giuliani in 2008 and another Bush, Jeb, in 2016.

That Trump spike four years ago is always worth noting. He wasn’t included in polls at this point in the 2016 cycle because it was expected that he, again, would decline to run. As you might remember from your history books: He ran.

So this doesn’t tell us a whole lot — but let’s introduce another comparison.

Here’s how the past five Democratic primary contests have looked at this point in the cycles.

The two candidates who led in April and went on to get the nominations were former vice president Al Gore in 2000 and Clinton in 2016. Each had a lead of at least 40 points at this point in the cycle.

The two times the leading candidates the prior April didn’t get the nomination, their leads were more modest. Lieberman led John F. Kerry by eight points and Clinton led Obama by 13. In each case, it was the second-best-polling candidate at this point that went on to win. Good news, in theory, for Sanders.

But we’re plagued here by a small sample size. The moral of this story isn’t that Biden is in a bad position; again, leading is generally better than not leading. The moral is that Biden’s lead is far from stable.

If recent Democratic Party primary history is any guide, it’s more likely that Biden’s narrow lead at this point won’t hold up than that it will.