Elizabeth Warren's campaign has grown to double the size of her Democratic rivals who are bringing in millions more in donations, raising questions about whether the Massachusetts senator will be able to garner enough small-dollar financial support to sustain her Boston-based operation through the long primary season.

By the end of March, Warren's campaign staff numbered about 164 people, according to payroll spending released this week in a quarterly Federal Election Commission disclosure. The 69-year-old candidate, who was the first major contender to jump in the race with a New Year's Eve announcement, now has a team of more than 170 people and plans to bring on new hires every month in the second quarter of 2019, campaign officials confirmed.

Early on in the race, more than nine months before primary voting begins, those figures make Warren's salary and payroll expenses significantly larger than those of her main rivals. FEC reports show that she spent more than $1.7 million on payroll and payroll taxes and fees.

As Warren and her advisers see it, it's part of a larger strategy that diverts from past presidential campaigns that have prioritized spending on television ads. As voters change the way they consume information online, they say, Warren has focused on building a campaign operation in early-voting states like Iowa and New Hampshire, holding events (58 town halls in 14 states) where the emphasis is on answering questions (more than 250 from audiences), engaging with the press (105 one-on-one interviews and 44 media availabilities), and demonstrating substance on policy.

“We are building a grassroots organization that’s built to last,” said Kristen Orthman, the campaign’s communications director. “We have front-loaded a tight-knit team and set our organizational plans, priorities, and culture faster and in finer detail than anyone.”

Vermont Sen. Bernie Sanders, who is leading early Democratic primary polling among declared candidates and raised more than $18 million in the first quarter, has built the second-largest campaign staff — with 86 people and more than $832,000 in payroll, according to a spokesperson.

The figures do not include payments to consulting firms.

Warren, who has released a bevy of detailed policy proposals and is maintaining a busy travel schedule to states that kick off the Democratic primary in February and March, raised more than $6 million in the first quarter of 2019, but spent $5.2 million, according to her campaign. It's a high burn rate for a top contender. Candidates hope to bring in more money than they are spending, particularly this early in the race.

Warren has made a point this year of swearing off the sort of high-dollar fundraisers she has held in past statewide races. Because of a transfer of $10.4 million from her Senate campaign account earlier this year, she began the second quarter of the year with about $11 million in the bank — a war chest that past presidential candidates who flamed out from high early spending never had to fall back on.

Still, the rest of the Democratic field is operating with campaigns that are far smaller than Warren's or Sanders'. FEC reports show that in the first quarter of the year, Cory Booker spent more than $890,000 in payroll on 62 staffers; Kirsten Gillibrand spent more than $867,000 on 43 staffers; Kamala Harris spent more than $706,000 on 44 staffers; John Delaney spent more than $673,000 on about 50 staffers; and Beto O'Rourke spent more than $161,000 on roughly 30 staffers.

Of the top-tier candidates, South Bend Mayor Pete Buttigieg had the smallest operation in the first quarter, with about 10 people and just $156,000 in payroll spending, though in the weeks since the Afghanistan war veteran seized the national spotlight, his aides have been working to add to their team in the early-voting states.

Tarini Parti contributed reporting to this story.