Joe Biden’s fund-raising strategy, focused on using high-dollar events to rake in money from donors who can cut big checks, has continued to pay off in Pennsylvania, where the former vice president was born and his 2020 presidential campaign is based.

But that approach is starting to show its limitations: Biden’s fund-raising momentum has slowed, other candidates have cut significantly into his lead in the Pennsylvania money race, and he does not have the wide base of small-dollar donors that some other candidates can call on for repeated contributions.

From when he announced his candidacy in April through the end of September, Biden brought in $1.4 million in large-dollar donations from Pennsylvania, more than double the total of any of his Democratic primary opponents, according to data recently released by the Federal Election Commission. Biden, a longtime U.S. senator from Delaware, spent his early years in Scranton. His campaign is based in Philadelphia and he’s held multiple fund-raisers in the state.

The data the campaigns reported to the FEC last week, though, show that Biden raised much less money in the last quarter than in the previous one, when he announced his candidacy — even though he was a candidate for the entirety of the third quarter and just part of the second quarter. The new data include only donations from contributors who gave more than $200 in total, making for an incomplete picture of campaign fund-raising, especially as candidates like Sens. Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont rely heavily on small donations that don’t have to be detailed to the FEC.

Still, most of the money raised by Biden is disclosed, given his focus on large-money donors.

Biden’s fund-raising is moving in the wrong direction

Biden brought in about half as much money from Pennsylvania donors in the last three months as he did in the first three of his campaign.

Compare that to Warren, Sanders, and Mayor Pete Buttigieg of South Bend, Ind., who all increased their contributions in the state.

Biden started his campaign with a bang — $6.3 million in the first 24 hours. That included a fund-raiser at the home of Comcast senior executive vice president David L. Cohen. Biden has kept a busy schedule of fund-raisers, holding multiple events in Pennsylvania, including one Tuesday near Scranton before a rally there Wednesday.

But given the finite number of people who are able and willing to give $2,800, the maximum allowed per donor for the primary, Biden could be running out of wealthy Pennsylvania donors.

“You’re a victim of your own success because of campaign finance limitations,” said Alan Kessler, a Philadelphia lawyer and one of Biden’s top Pennsylvania fund-raisers. “If you do the incredible job that I think we did in the first fund-raiser, all who maxed out, they can’t write another check.”

Kessler said supporters are focused on getting those “maxed out” donors to bring in others.

Biden supporters say there’s still room to grow.

“There remains enthusiasm in the donor base,” Cohen said. “Even people who’ve decided they want to raise for someone else still want to hedge their bets and support Joe Biden.”

Nationally, Biden’s large-dollar fund-raising fell by about 21% from the second quarter to the third. But in Pennsylvania, the drop was even steeper, at 52%.

By contrast, most of Biden’s major rivals saw growth in their big-money fund-raising, including a 103% increase for Sanders, 54% for Warren, 13% for Buttigieg, and 73% for Sen. Kamala Harris of California. As a result, the gap between Biden and the other candidates shrank dramatically.

Look at it another way: Of all the big-dollar money that Pennsylvania donors gave to campaigns in the second quarter, slightly more than half of it went to Biden. Last quarter, that number fell to 22%.

Part of the increase in large-dollar donations for Sanders and Warren comes from donors who give small donations on a regular basis and eventually hit the $200 threshold, at which point those donations are reported. Both Sanders and Warren have made it a point to not hold formal fund-raisers for large donors.

“What you’re seeing is, we have a lot of people who give multiple donations,” said Joe Calvello, a spokesperson for Sanders’ campaign. “They may give $10 here, $20 there, and over the course of the campaign, that adds up to above $200.”

Other candidates are starting to eye big donors in Philadelphia. Harris’ campaign held a September fund-raiser in the city, and Buttigieg greeted donors at a $2,800-per-ticket fund-raiser before his rally there last weekend.

How Biden’s campaign is responding

Big money used to be the name of the game. But small money has become a major force in politics, with Sanders’ 2016 campaign demonstrating the power of many donors giving smaller amounts.

The rise of easy online giving has also helped fuel the rise of small donations, and the Democratic National Committee’s rules for qualifying for debates — requiring candidates to receive money from a certain number of donors — has emphasized breadth of donor pools.

Biden’s campaign has recognized the need to increase his small-dollar donations, sending frequent emails and text messages seeking donations as small as $5.

“We know we have your trust, we know we have the polls, but small-dollar grassroots donors are a critical measure of strength, and we can’t fall behind,” read a campaign text message from Oct. 17, shortly after the FEC reports came out. “If we can’t catch up, we won’t be able to fund critical voter turnout operations, hire field staff, or run our crucial TV ads in the early states.”

“You can’t just rely on fund-raisers,” Kessler said. “For the simple reason there’s a maximum of the people who can contribute, you’ve got to work on that internet small-dollar fund-raising.”

Cohen said the campaign is showing early signs of success, raising $1 million in small-dollar donations in the first week of October.