WASHINGTON — Bernie Sanders proved this week he can fill his bank account as fast as he can fill an arena. 

The challenge now for the independent senator from Vermont is to convert all that money — he has $27 million to spend — into a winning campaign for president.

That won’t be easy in a Democratic race against Hillary Rodham Clinton, an experienced White House candidate who is significantly ahead in the buildout of her operations, as financial reports filed Thursday with federal regulators show.

Clinton, the former secretary of state and senator from New York, has about five times as many employees as does Sanders, as well as offices across the country and powerful voter research tools already in place.

Sanders only recently hired a pollster.

While Clinton has already dropped $6 million on ads, more than half on television, Sanders has so far relied only on digital advertising. He plans to air his first TV ads around Thanksgiving, roughly three months after Clinton began putting spots on the air.

“They have spent millions of dollars that we have not had to spend because of the power of our message and the power of our candidate,” said Jeff Weaver, Sanders’ campaign manager.

The excitement generated by Sanders’ campaign has, to date, largely been reflected in packed arenas and auditoriums, where the self-described democratic socialist has promised to foster a “political revolution.”

But the reports filed with the Federal Election Commission show that he’s doing more than just drawing people to his events. Over the summer months, Sanders kept pace with Clinton’s fundraising, collecting more than $26 million from July 1 to Sept. 30, a time when Clinton raised $29 million.

The FEC documents reveal stark differences in how they did it.

Three-quarters of Sanders’ contributions were for $200 or less, though he did net about 300 checks of $2,700, the legal maximum, from donors including actor Danny DeVito, Silicon Valley venture capitalist Andrew Rappaport and New York entrepreneur Henry Jarecki.

People giving less than $200 made up just 17 percent of Clinton’s summer fundraising.

Instead, she leaned heavily on traditional financing events such as dinners where people must pay $2,700 to attend. Her campaign on Thursday also released a lengthy list of its “Hillblazers,” or fundraisers who have bundled at least $100,000 in donations from others for the campaign since its launch in April.

Among those top fundraisers: Virginia Sen. Tim Kaine, and Reps. Grace Meng of New York, Jim Himes of Connecticut and Joaquin Castro of Texas.

“Thanks to the support of all of our donors, we are on track to hit our goal of $100 million during the primary, which will set us up to reach more voters, organize them, and make Hillary Clinton the Democratic nominee in 2016,” Robby Mook, Clinton’s campaign manager, said in a statement.

Clinton is quickly burning through much of what she brings in — about 88 percent of what she raised in the third quarter. Sanders spent less than half of what he’d raised.

But Clinton’s aides say they budgeted for a high level of spending at this stage of the race, arguing they are making important investments in data analytics, digital infrastructure and organizing that will to help them win both the primary and the general election.

For example, in Nevada, the fourth state to cast ballots in the presidential primary season, Clinton installed staff on the ground six months ago and has expanded her team there to 22 paid operatives. They have recruited more than 3,000 volunteers, who have already held events across the state.

Sanders, meanwhile, put his first staffer in the state only a few weeks ago.

Clinton has also expanded her investment in the so-called Super Tuesday elections in an effort to build a March 1 firewall, should Sanders pick up wins in early-to-vote Iowa and New Hampshire. Data analyzed by President Barack Obama’s campaign showed a direct correlation between supporter enthusiasm in the last six weeks before the election and how early local organizing efforts began in their area. Some of Obama’s campaign staff now work for Clinton.

All told, by the end of September, Clinton was paying 511 staffers, which cost her campaign about $8.5 million over the three months. Salaries in Sanders’ campaign came in at about $1 million in that time.

While payroll was Clinton’s greatest expense, for Sanders it was “campaign paraphernalia,” FEC documents show. Most of that went to Tigereye Promotions LLC, which says online that it is “a union shop specializing in promotional products,” such as T-shirts.

Sanders’ no-frills approach to the race showed up in his spending report. His campaign paid for about $4,000 in room reservations through Airbnb, an Internet sharing service that’s often cheaper than hotel stays, and he uniformly traveled on commercial airlines.

Clinton spent $560,000 on chartered planes and $10,000 at Four Seasons hotels, her campaign documents show.

Each campaign ended last month with plenty of available cash. Clinton had $32 million and Sanders $27 million — totals higher than any of the candidates on the Republican side.

And there are signs that Sanders can continue to count on a cash influx.

In the hours after Tuesday’s Democratic debate, he raised upward of $2.5 million. That’s more than Kentucky Sen. Rand Paul raised all summer for his Republican presidential bid.