Biden and Sanders have shown remarkable polling consistency. Photo: Justin Sullivan/Getty Images

A lot has changed in the Democratic presidential primary over the course of the past year. On the ground in Iowa and New Hampshire, Elizabeth Warren and Pete Buttigieg have upended early primary conventional wisdom about front-runners and up-and-comers. Once-promising candidates like Beto O’Rourke and Kamala Harris have faded away. Now, just a few weeks out from voting, the contest looks as unpredictable as any in recent memory — that much is obvious to anyone who’s paid any attention at all in 2019.

But a lot also hasn’t changed. While there’s no single national primary, and country-wide polling could very easily once again prove especially responsive to results in the early voting states, it can still tell us plenty about candidates’ strength and the shape of their support. And when it comes to Joe Biden and Bernie Sanders — who yet again sailed through a debate on Thursday night without facing any direct, sustained attacks from their opponents — it looks a lot like we’re back to where we started.

Here’s the top of the RealClearPolitics national polling average on December 19, 2018, before any of the major candidates were even in the race:

Biden - 27.5

Sanders - 19.0

And here’s that same average on December 19, 2019:

Biden - 27.8

Sanders - 19.3

Obviously the numbers below Biden and Sanders have shifted — in 2018, the next two contenders were Warren and Harris, both at 5.0, and now it’s Warren at 15.2 and Buttigieg at 8.3. There was also not a lot of quality polling back in December 2018. (FiveThirtyEight’s national average only goes back to mid-March, when Biden was 2.6 points higher than he is now and Sanders 3.2 higher.) And yes, Biden’s and Sanders’s support levels and the demographic makeup of their backers have moved around over the course of the year before landing here. Could Iowans turn this into little more than a weird observation when they caucus in February? Definitely.

But it’s also clear that for all the talk of historic volatility and an overcrowded field, the two best-known candidates have benefited from some extraordinarily persistent dynamics that may be harder to dislodge than widely acknowledged — especially if they’re still standing in March, once the election broadens past the four early voting states into more of a national campaign.