The candidates who are still courting big-money donors — Ms. Harris, Cory Booker, Kirsten Gillibrand, Amy Klobuchar — are doing so on the down-low. High-dollar fund-raising is the one part of the primary that in the 2020 race has actually become more invisible.

High-profile endorsements don’t carry the same weight as they once did, either. The power of superdelegates — elected officials and party leaders who voted as they wished at the presidential convention — has been greatly reduced in the 2020 campaign. In response to Mr. Sanders’s complaints about Mrs. Clinton’s superdelegate advantage in 2016, the Democratic National Committee last year adopted rules that forbid superdelegates from casting the deciding vote on the first ballot at the convention. (If no candidate wins a majority in the first round, superdelegates are permitted to vote in subsequent rounds.) As a result of this rule change, a crucial aspect of the old invisible primary — courting elected officials for their endorsements — is less important in 2020.

And these endorsements could actually carry a stigma. “The way we thought endorsements mattered before was, to the extent everyone in the party was for someone, you got the sense that this person was the party’s choice,” said Hans Noel, a Georgetown political science professor and a co-author of “The Party Decides.” “But it’s possible no one’s going to want to claim that mantle now, because of the sense that there was something unfair about the way Clinton had all that support in 2016.”

Even the way candidates build their campaign staffs has changed. Before, candidates competed with one another to make splashy hires of campaign managers and media strategists in the hopes of impressing donors, endorsers and, often, political reporters. In the 2004 race, for instance, the invisible-primary competition between John Kerry and John Edwards for the services of the veteran media strategist Bob Shrum was called the “Shrum primary.” Mr. Kerry won the Shrum primary, which in turn helped him win the invisible primary, which ultimately won him the nomination.

But in 2020, said Tommy Vietor, a co-host of the “Pod Save America” podcast and a former Obama campaign staff worker, “the era of headhunting for the biggest, best consultant is over. The nature of your whole communications staff has changed. The communications team I was on in 2008 was almost exclusively designed to handle reporters. A communications team in 2020 is designed to go around the filter. Well over half if not three-quarters of the work it does is creating its own media, whether it’s live stream, memes, podcasts, whatever.”

Democratic rank-and-file voters are receiving some of the same treatment — or at least a simulacrum of that treatment — that presidential candidates once reserved for party elites. “Voters are incredibly hungry for authenticity and believability, and it’s critically important that they see who the candidate really is,” said Jesse Ferguson, a Democratic strategist who worked on Mrs. Clinton’s 2016 campaign. “You don’t get that from a 5,000-person rally, but you might get that from a day on the road with a videographer on Instagram.” Ms. Gillibrand has gone so far as to hire two campaign workers who, as The Washington Post reported, “make personalized appeals” to her online supporters.

The transformed primary has led to a revaluing of resources. Out are bundlers and endorsers. In are ears and eyeballs. “There’s intense competition for attention,” said Peter Hamby, the host of Snapchat’s “Good Luck America.” “Attention is now the main currency in our politics.”