Ms. Warren collected money in the second quarter from more than 384,000 donors, whose donations averaged $28, her campaign said. She finished the quarter with $19.7 million in cash on hand, less than $100,000 of which is earmarked for the general election, her campaign said.

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Ms. Warren’s decision to rely on small donations to power her campaign still carries risk, given that some of her rivals are raking in huge amounts of cash at fund-raising events with wealthy donors.

She has enjoyed a big financial cushion in the early stages of the race because she transferred $10.4 million to her presidential bid from her Senate campaign account in the first quarter.

But she will need to keep raising money to pay for the expensive campaign operation she has built. Her campaign’s spending in the first quarter of the year, more than $5 million, was the highest in the Democratic field. She had by far the biggest staff in that quarter, according to Federal Election Commission records, with about 160 people on her payroll.

Her staff has grown to 300 since then, her aides said, with 60 percent of her employees stationed in Iowa, New Hampshire, Nevada and South Carolina, the first states to vote in next year’s primary. She has built a particularly large operation in Iowa, the first state with a nominating contest.

Instead of courting wealthy donors, Ms. Warren has made a habit of placing phone calls to people who give her money online, interactions that have ended with joyful posts on social media. At her campaign events, she sticks around to take pictures with anyone who wants to meet her.

Presidential candidates are required to file their campaign finance reports with the Federal Election Commission by July 15. Those reports will provide further details on how the candidates are raising and spending money — including a more detailed picture of how Ms. Warren has expanded her campaign operation since the last quarter.