Biden leads Sanders by 19 points in FiveThirtyEight's national Democratic primary poll average as of 2020-03-30, basically unchanged over several weeks.

True: FiveThirtyEight's national Democratic primary polling average on or about 2020-03-30 showed Biden leading Sanders by about 19 points (18–20) and the lead had stayed roughly stable over the previous several weeks. Mostly true: the lead was within about 2 points of 19, or it was 19 but had changed somewhat over those weeks. Mixed: the lead figure is right but it clearly was not stable, or the reverse. Mostly false: the lead differed by 3–6 points. False: the lead differed by more than 6 points, or Biden did not lead.

Set before the research began.

1. E1 Joe Biden wins the Arizona primary Vox · 17 March 2020

   “As of March 17, FiveThirtyEight’s average of national polling data showed Biden with a whopping 25-point lead and with about 52 percent support.”

2. E2 Joe Biden up to 96% chance of winning nomination in FiveThirtyEight forecast Raw Story · 10 March 2020

   “Polls similarly show Biden leading Sanders by an average of 25 points, according to RealClearPolitics.”

3. E3 2020 Democratic Presidential Nomination | RealClearPolling RealClearPolling

   “| RCP Average | 3/29 - 4/7 | — | 60.8 | 32.0 | Biden+28.8 |”

4. E4 2020 Democratic Presidential Nomination | RealClearPolling RealClearPolling

   “| Reuters/Ipsos | 2/28 - 3/2 | 469 RV | 18 | 29 | Sanders+11 |”

5. E5 2020 Democratic Presidential Nomination | RealClearPolling RealClearPolling

   “| Reuters/Ipsos | 3/18 - 3/24 | 1981 RV | 53 | 34 | Biden+19 |”

6. E6 Wisconsin primary election: April 7, 2020 Vox · 7 April 2020

   “Heading into the primary election, Biden has held a firm lead over Sanders, according to FiveThirtyEight’s average of polls, which had the former vice president at 53 percent support and the Vermont senator at 36 percent.”

7. E7 DemDaily: Presidential Pause and Polls - DemList DemList · 31 March 2020

   “According to a *RealClearPolitics* average of six national polls conducted between March 18-March 30, 2020 of Democratic primary voters, former Vice President Joe Biden is leading Bernie Sanders 56.3% to 35.3%.”

8. E8 Infographic: Biden Has Clear Lead Over Sanders, Trump Statista · 31 March 2020

   “In an aggregate of new March polls, Biden is leading Sanders by 21 percentage points with just a couple months before the Democratic National Convention.”

9. E9 Dems' Roller Coaster Steadies as Center & Center-Left Take Over | RealClearPolitics RealClearPolitics · 18 March 2020

   “With his impressive win in Nevada, Sanders’ lead grew to 12.2 points the day after the primary.”

None of the evidence gives FiveThirtyEight's national average on or about 2020-03-30. E1 reports a 25-point Biden lead in that average on March 17, and E6 reports 53% to 36% (17 points) heading into the April 7 Wisconsin primary. Those figures bracket the claimed 19 points but do not confirm it, and filling in the March 30 value from them would be a guess. The other items are RealClearPolitics averages, other aggregates or single polls (E2, E3, E5, E7, E8), not FiveThirtyEight's average. The FiveThirtyEight figures in the evidence (a 25-point lead on March 17 [E1] and 53 to 36 percent on April 7 [E6]) fall either side of the claim date. They do not show the March 30 value or whether the lead held steady in the weeks before it, so neither the 19-point figure nor its stability can be checked against the evidence.