Senator Bernie Sanders would begin a 2020 presidential bid with 2.1 million online donors, a massive lead among low-dollar contributors that is roughly equivalent to the donor base of all the other Democratic hopefuls combined.

Beto O’Rourke, the former Texas congressman who narrowly lost a Senate race last year, is also poised to be a fund-raising phenom if he runs for president: He has twice as many online donors as anyone eyeing the race besides Mr. Sanders.

Three senators who are already running have their own solid track records with small donors. Senator Elizabeth Warren, with the third-highest number, has notable strength in New Hampshire, even topping Mr. O’Rourke there. Senator Kirsten Gillibrand has built up broad national support among small donors, despite a reputation as a big-money fund-raiser, while Senator Kamala Harris raised $1.5 million online in her first 24 hours as a presidential candidate.

Small-dollar donations are expected to be a huge deal in 2020 — the renewable resource that Democratic candidates will depend upon to fuel their campaigns. And those five Democrats represent a distinctive top tier with the most formidable followings, each counting a base of at least 230,000 online donors, according to a New York Times analysis of six years of federal election filings from ActBlue, the Democratic Party’s dominant donation-processing platform.

[Check out the 6 days when 2020 Democratic hopefuls scored big with small donors.]

The findings provide a window into one of the most closely guarded and coveted resources of a modern campaign: the digital donor lists that bring in the vast bulk of low-dollar donations. These online donations average just under $40, and candidates like to point to such modest amounts as evidence of the breadth and depth of their support among regular people.

In the early stages of a presidential race, when polling measures little more than name recognition, the relative size of donor networks can provide one of the best metrics of strength.

“The people who have a strong base right now have a material head start,” said Teddy Goff, who served as a top digital strategist for the campaigns of President Barack Obama in 2012 and Hillary Clinton in 2016. “And more often than not, there is a good reason they have that base, and it’s that they have a talent for connecting with the grass roots of our party.”

For Mr. Sanders and Mr. O’Rourke, the enormous early edge in their donor rolls has afforded them the flexibility to wait longer before deciding to jump in, and has sparked a sense of urgency in other campaigns. Both men have signaled they would rely overwhelmingly on small donors to fuel any campaign.

[Make sense of the people, issues and ideas shaping American politics with our newsletter.]

The particular power of Mr. Sanders’s list was on display in late December when he emailed supporters with the provocative subject line, “If I run.” That single email netted $299,000 from 11,000 donations, according to a senior Sanders official.

“It is hard to see someone winning this nomination who isn’t at or near the top of ability to generate small donations, because they are a measure of enthusiasm,” said Anita Dunn, a Democratic strategist and veteran of the Obama White House.

Candidates are under intense pressure to lock down donors, big and small, in an already sprawling race. Whether small-dollar donors and larger bundlers give to multiple candidates or stay “monogamous” remains an open question, as Democrats have not had such a sizable field since the rise of online fund-raising.

The findings represent the best snapshot of small donor strength at the starting gate of a campaign where the ability to raise tens of millions of dollars from small donors will be crucial, particularly as leading candidates are disavowing super PACs that allow for unlimited sums from wealthy backers.

Of course, in a race likely to stretch over the next 18 months, early advantages can dissipate quickly. A strong poll or viral moment can prompt donors to give to new candidates, thereby growing their lists by huge numbers — so long as they are positioned to capitalize. Ms. Harris’s launch-day haul, for instance, rivaled that of Mr. Sanders in 2015.

Ms. Dunn said she saw the initial rankings less as a predictor for the coming primary’s outcome and more as a revealing indicator of “who was able to use their 2017 and 2018 effectively to prepare for a presidential race.”

Both Ms. Harris and Ms. Gillibrand landed on the leader board despite having not faced a competitive election in recent years. The two senators spent heavily to bulk up their small donor lists, investing in multimillion-dollar campaigns on Facebook in 2017 and 2018 to add email addresses to their supporter list and lure in new contributors.

Mr. Booker is among the most widely followed potential presidential candidates on social media (he has 4 million Twitter followers) but the New Jersey Democrat had the seventh-most donors among senators looking at 2020 bids. Mr. Booker’s campaign website intermittently used ActBlue in 2013, 2014 and 2015, suggesting his full number of online donors is likely at least somewhat higher.

“He’s the perfect case study to show what it takes to build a large online grass-roots following today. It’s folly to think you can just grow that organically,” said Mr. Lim, the digital strategist. “Just because you have a lot of followers or people who like you or talk to you, that doesn’t translate to an organized and impactful community of donors.”

Mr. Booker has announced that Jenna Lowenstein, a former top digital strategist for Mrs. Clinton’s 2016 campaign, would serve as his deputy campaign manager, a move seen as focusing on improving his online donor footprint.

Senator Amy Klobuchar of Minnesota, who is expected to announce a 2020 bid on Sunday, lags far behind her colleagues, with fewer than 38,000 donors. That figure was about on par with Richard Ojeda, who briefly declared his presidential candidacy after losing a House race in West Virginia before abruptly quitting in late January.

Representative Tulsi Gabbard of Hawaii, who announced her bid for president in January, had 41,000 donors, but more than 75 percent of her donors overlapped either with Mr. Sanders, whom she endorsed in 2016, or with at least two other potential candidates.

Ms. Gabbard’s single best day for online donations came two and half years ago. That was the day Mr. Sanders sent a fund-raising email for her.