Democratic presidential candidate Joe Biden is introduced before speaking at an event in Dallas on May 29. (Smiley N. Pool/Dallas Morning News/AP)

From August 2015 until he locked up the Republican nomination the following year, Donald Trump never had less than 20 percent support in an average of Republican Party primary polling. By November, his approval never again sank below 25 percent. In a field with 16 other major candidates, that stability was a huge asset, locking up enough of the vote to ensure top-tier finishes in early primary states.

Before Trump’s emergence in 2015, no Republican had done better than 20 percent. It was a mishmash of various candidates, a truly open field. Then Trump stepped in and, powered by his conservative-media-friendly rhetoric, started solidifying things.

On Tuesday, CNN and its polling partners at SSRS released a new poll of the 2020 Democratic primary field. Former vice president Joe Biden has the support of 32 percent of Democrats, trailed by Sen. Bernie Sanders (I-Vt.) with 18 percent. No other candidate is in the double-digits.

Biden’s position relative to Sanders has softened somewhat since the last poll conducted by CNN. In late April, he was 24 points ahead of Sanders. In this most recent poll, completed at the end of last month, he’s only up 14.

(Philip Bump/The Washington Post)

But that comports with the RealClearPolitics average of polls. Biden saw a surge after he got into the race in late April, a surge that waned a bit and left him about where CNN has him now. (The figures below don’t include the new CNN numbers.)

(Philip Bump/The Washington Post)

CNN asked an additional question that offers similarly good news for Biden: Who is your second choice? Biden was the second pick of 16 percent of respondents, meaning that nearly half of Democrats had him as their first or second choice. (Sen. Elizabeth Warren was the second-most-chosen backup candidate, with 14 percent.)

(Philip Bump/The Washington Post)

Remember: This is in a field with 23 major candidates.

It’s not just that Biden is well-known among Democrats. He’s also well-liked. His net favorability (favorable rating minus unfavorable rating) among members of his own party is plus-64, about equal to Sanders’s plus-63.

Why does Biden do so well in this poll? Largely for the same reason he’s done well in other polls: solid support from more moderate Democrats while his opponents fight for support from liberals. In the CNN-SSRS poll, Biden does 13 points better with moderates than liberals, while Sanders does 11 points better with liberals.

(Philip Bump/The Washington Post)

Biden and Sanders also fare better with Democrats who don’t have a college degree than do Warren, Sen. Kamala D. Harris (D-Calif.) and South Bend, Ind., Mayor Pete Buttigieg.

(Philip Bump/The Washington Post)

Sanders does better with lower-income Democrats, while Harris and Biden do better with higher-income ones.

(Philip Bump/The Washington Post)

But that effect for Sanders may correlate to the fact that he does much better with younger voters (who tend to have lower incomes). Biden, on the other hand, does much better with older ones. Among voters under 45, Sanders wins by 7 points. Among those 45 or older, Biden wins by 35.

(Philip Bump/The Washington Post)

We can’t and this poll doesn’t predict the future, of course. But it’s a reminder of the enviable position in which Biden finds himself: with strong support from moderates and older Democrats that might similarly help him power through early primary states.

We’ve written before about one of the key factors powering Biden’s success. Democrats appear to think he’s got the best chance at beating Trump, something that’s of more concern to older voters. We’ve also pointed out that voters aren’t actually that great at predicting electability and that part of the assumption that Biden is more electable is rooted in the idea that the Democrats need to win the race that Hillary Clinton lost in 2016. But regardless, it’s a powerful motivation.

It was interesting to watch over the weekend as a number of leading contenders visited a party event in California and focused their aim on Biden. Most of the critiques that were leveled against the former vice president were ones that centered on his more-moderate policy positions — a pitch that would probably appeal to party activists in a heavily blue state. But it’s not clear that this line of attack will do much to undercut either the support Biden has because of electability or his strong support from more-moderate members of the party.

To some extent, this electability idea seems to be self-reinforcing: It helps Biden stay in the lead, which helps advance the idea he’s got a better shot at winning.

It’s up to his competitors to break that rhythm. In 2016, no Republican was able to similarly dislodge Trump.