Joe Biden's high-dollar donor strategy wasn't the best fundraising return on investment over the summer, but it's the only option he's got, Democratic strategists said.

Biden raised $15.2 million during 2019's third financial quarter, well short of Bernie Sanders' $25.3 million, Elizabeth Warren's $24.6 million, and Pete Buttigieg's $19.1 million from July through September. Warren's haul, up from $19.1 million in quarter two, despite summer being a harder time to bring in money, projects momentum heading into fall as she catches up with the former vice president, the Democratic primary's front-runner, in the polls.

Sanders and Warren, the senators from Vermont and Massachusetts respectively, have shunned private fundraisers in favor of small-dollar donations, drummed up with the help of their powerful email databases. Warren shares videos on social media of her thank you calls to contributors.

"I'm not spending my time behind closed doors with a bunch of millionaires," Warren said in June.

Despite the conversation from his White House rivals, Biden's persevered with the traditional fundraising model. He's spent hours of campaign time compared with Sanders and Warren rubbing shoulders with donors at events with up to $2,800 ticket prices, the maximum amount people can give federal primary candidates. Although the mayor of South Bend, Indiana, also hosts exclusive fundraisers, gatherings for Delaware's senator for 36 years draw extra scrutiny since his campaign invites select reporters who send out an account of what happened to other members of the press.

In September alone, Biden spoke at almost two dozen fundraisers held at ritzy locations around the country: palatial homes, upscale restaurants, and event spaces in cities such as Los Angeles, Miami, Houston, and Utah's Park City, and even communities including Sullivan's Island in South Carolina. Actors Alyssa Milano and Zach Braff attended an event in the Bel Air neighborhood of Los Angeles.

Concerned about the perception, the Biden campaign insisted "grassroots fundraising remains central to the campaign’s growth with 98% of all donations coming from grassroots donors of less than or equal to $200."

"This quarter, 56% of donors were new to our campaign. Additionally, 70% of all donors to this campaign are now individuals who were not previously known to us when the campaign launched. The campaign has seen an influx of support along with an average donation of $44, reinforcing grassroots support as fuel for the campaign," the team wrote in a statement announcing their efforts.

While he's received criticism and trailed Sanders and Warren in third-quarter results, Biden's lack of digital infrastructure leaves him with little choice but to rely on high-dollar contributors, one Democratic campaign fundraising tech service provider told the Washington Examiner.

"Biden was just a vice president. He had this job for eight years that built a public profile but didn't build an email list that went with it. So they were basically starting from scratch when he announced for president," the service provider said, adding that Biden's campaign wasn't given former President Barack Obama's massive email database.

Acknowledging email fundraising can be "fickle," the Democratic fundraising expert, who wanted anonymity to speak candidly, said he believed Biden's better performance of $22 million in the second quarter was easily explained. The former vice president's posting from April through June was behind Buttigieg's $24.8 million, but ahead of Warren's $19.1 million and Sanders' $18 million.

"He just lined up all these donors that were going to give him the maximum amount, and they had that all lined up well in advance of him announcing. The problem is that so many of those people gave him the max, they can't give him more money," they said. "Things are not looking up for Biden."

Building an email list takes time because of Google, Yahoo, and AOL algorithms, though a candidate can boost their open rate and avoid having their notes being filtered as spam if they're attached to viral news, according to the expert.

Tom Cochran, a partner at public affairs firm 720 Strategies, said that it doesn't really matter where the money comes from as long as the campaign continues to raise funds, but it's helpful to be able to say it's from the grassroots because that indicates a groundswell of support. The downside, however, is that Biden has to work harder to keep pace with Sanders and Warren.

Warren also has the advantage of having Joe Rospars, Obama's 2008 and 2012 digital strategist, on staff, Cochran told the Washington Examiner.

"It's not great if your numbers are less than the other candidates', especially if you're the front-runner. It's not good. At the end, it really is about money, for better or worse," he said.

For public affairs firm Summers Strategies' Matt Fuehrmeyer, grassroots contributors are nevertheless crucial because they signify enthusiasm.

"Those small-dollar donors are important not just for the money they can bring in on multiple occasions, but those are individual humans who have skin in the game for your campaign. They're potential volunteers on the ground, they're potential canvassers or people making phone calls," Fuehrmeyer said. "To a teacher in Iowa, $100 a month means a heck of a lot more out of their wallet than a $2,800 check from some CEO. Campaigns cash both checks because all that money spends the same and they value all of the donors that they get, but those small-dollar donors are more than just dollar signs to campaigns."

He added, "The extent to which you concentrate on one or the other is a choice that you have to make as a campaign based on where the money is. It's the same reason you rob banks."