With the first debates looming later this month, the 2020 Democratic primary polls are stubbornly stable.

Former Vice President Joe Biden holds a sizable lead, and Sen. Bernie Sanders (I-VT) has fallen back to a pretty distant second. Sens. Elizabeth Warren (D-MA) and Kamala Harris (D-CA) sit in third and fourth; the order depends on the survey. South Bend, Indiana, Mayor Pete Buttigieg, former Rep. Beto O’Rourke (D-TX), and Sen. Cory Booker (D-NJ) are the only other candidates consistently pulling noteworthy support. Everybody else is stuck at 1 or 2 percent.

Two new national polls, from CNN and Morning Consult, stand out if only for how similar they are and how little has changed over the past few months. First, from Morning Consult, a poll of 16,600 registered voters from May 27 to June 2:

Joe Biden: 38 percent

Bernie Sanders: 19 percent

Elizabeth Warren: 10 percent

Pete Buttigieg: 7 percent

Kamala Harris: 7 percent

Beto O’Rourke: 4 percent

Cory Booker: 3 percent

Then from CNN, surveying 1,000 voters from May 28 to 31:

Joe Biden: 32 percent

Bernie Sanders: 18 percent

Kamala Harris: 8 percent

Elizabeth Warren: 7 percent

Pete Buttigieg: 5 percent

Beto O’Rourke: 5 percent

Cory Booker: 3 percent

Sen. Amy Klobuchar (D-MN): 2 percent

Former HUD secretary and San Antonio Mayor Julián Castro: 2 percent

Accounting for margin of error, these polls are basically identical. Biden has been in the race for a month, and despite ample media and competitor skepticism that he can maintain his lead, he hasn’t really flagged at all so far. The debates, putting every candidate on the same stage and drawing a national TV audience, could change that.

Biden’s lead does not appear quite daunting in some of the early primary states, however. The RealClearPolitics average of Iowa polls gives him a 4-point lead there over Sanders (24 percent versus 20 percent). In New Hampshire, he’s averaging a 13-point lead, but polling has been volatile, with Sanders occasionally coming out ahead. Biden does seem to have a significant advantage in South Carolina, currently holding an average 24-point lead.

What explains Biden’s persistent lead? Older people.

The most interesting demographic in the 2020 election polling, explained

Piecing through the smaller data sets in the CNN poll — how men, women, whites, nonwhites, college grads, and non-college grads are leaning — one trait sticks out more than the rest: age.

Here is how Democratic voters over age 45 break down, according to CNN:

Joe Biden: 45 percent

Bernie Sanders: 10 percent

Elizabeth Warren: 8 percent

Kamala Harris: 7 percent

And here is how the candidates fear among Democratic voters under age 45:

Bernie Sanders: 26 percent

Joe Biden: 19 percent

Kamala Harris: 9 percent

Elizabeth Warren: 7 percent

The sample sizes in these polls do get perilously small when you start breaking down respondents by demographic factors like age. But the CNN finding tracks with what we’ve seen from other national polls.

A recent Quinnipiac survey showed Biden leading Sanders by a whopping 38 points (46 percent to 8 percent) among voters over 50, but they were tied at 23 percent among voters ages 18 to 49. A Monmouth poll found 40 percent of Democratic voters over 50 backing Biden; Sanders was all the way down at 4 percent with those voters, while Warren and Harris had 10 and 11 percent, respectively. Among the voters under 50, though, Biden (26 percent) and Sanders (25 percent) were effectively tied.

Age is emerging as the most important dividing line in the Democratic primary. In the CNN poll, men, women, whites, and nonwhites all followed roughly the same trends: Biden in the lead at 30 or so percent, Sanders 10 or so points behind him, and then a cluster of other candidates. Age and, to a much lesser extent, education were the only areas that meaningfully deviated from the top line.

It’s Biden versus the field ... for now

The Democratic primary is in a holding pattern for now: Biden is well in the lead and then the names behind him keep shuffling, varying from poll to poll, with Sanders usually in second and Warren and Harris just behind him.

The primary debates, scheduled to start this month, could start to shake things up, when a national television audience finally gets their chance to see the candidates onstage together. We saw Sanders and Warren start to take more pointed jabs at the frontrunner at the California Democratic convention over the weekend, a sign of things to come.

The rest of the Democratic field is breaking down into Biden rivals and Biden replacements, as Kyle Kondik at the University of Virginia’s Center for Politics explained to me recently. Sanders and Warren lead the Biden rival camp; Pete Buttigieg and Beto O’Rourke are pretty clearly running as replacements. Harris is somewhere in between, a generational contrast to the vice president who has endorsed progressive policies like Medicare-for-all but who also steers clear of fierce ideological rhetoric.

“The Biden rivals are the candidates who hope to emerge as the main alternative to Biden and who can mobilize the support of younger, more liberal voters,” Kondik said. “The Biden replacements are those who seek to supplant Biden as a candidate who can rally the more moderate/older elements of the party.”

So we wait and see. As the Washington Post’s Dave Weigel reported, the other Democratic campaigns are skeptical that Biden’s frontrunner status will last, and voters give them good reason to believe that:

Donna Duvall, 70, was still looking for an alternative to Biden, whom she’d seen in Dubuque three weeks earlier, and who left her a little cold. “He had a nice presentation, but I didn’t feel like it was very energetic and committed,” Duvall said. “I didn’t think he had a lot of specific proposals; I like [Massachusetts Sen.] Elizabeth Warren a lot, and she’s made a lot of specific proposals. I loved Joe as vice president, but I feel like he’s maybe somebody whose time has passed.” The hopes of 21 Democrats — everyone in the field except Biden and Vermont Sen. Bernie Sanders, who are No. 1 and 2 in most polls — lie with voters like Duvall.

That’s why so many candidates have entered the campaign: They don’t believe Biden is a dominant frontrunner. If he falters, it becomes a wide-open race that anybody can win.