Democratic Superdelegate Rule Changes for 2020
Unpledged delegates, better known as superdelegates* will make up about 16% of Democratic Party delegates in 2020. These party insiders are part of each state's delegation, but are not committed to vote based on the outcome of the state's primary or caucus. This has caused considerable friction over the years. This came to a head in 2016 when many superdelegates announced early support for Hillary Clinton, creating the perception that the party was putting its thumb on the scale in her favor against Bernie Sanders.
As a result, the party has made a significant change for 2020. Superdelegates will no longer vote on the first ballot at the convention unless there is no doubt about the outcome. To win on the first ballot, the frontrunner must secure the majority of pledged delegates available during the nominating contests (primary and caucus) leading up to the Democratic Convention. There are 3,979 total pledged delegates, with the total required being 1,991. (Here's why it's not 1,990.)
All delegates become unpledged, with an estimated 771 superdelegate votes coming into play if the convention is contested (i.e., more than one ballot is needed to select a nominee). For those subsequent ballots, a majority of all 4,750 delegates (2,375.5) will be needed to secure the nomination. Given the large field, front-loaded calendar and the party's proportional allocation process, a contested convention can't be ruled out.