(CNN) The Democratic presidential contenders are racing to refill their campaign coffers -- following costly battles in the early nominating states and the looming Super Tuesday threat from New York tycoon Michael Bloomberg , who has plowed nearly half a billion dollars of his fortune into the fight to become the party's standard-bearer.

The campaign filings rolling into the Federal Election Commission on Thursday evening underscored the sharp money divide in the Democratic campaign as the race moves into an expensive coast-to-coast battle for delegates

Sen. Elizabeth Warren of Massachusetts spent more than twice what she raised in January and took out a $3 million line of credit to ward off running out of cash, her campaign reported late Thursday to federal regulators. And Pete Buttigieg on Thursday made his own urgent plea to supporters to help him raise $13 million -- or roughly $1 million a day -- before Super Tuesday's contests in 14 states after collecting just $6.16 million in January.

Aside from the billionaires in the Democratic primary, Sen. Bernie Sanders of Vermont entered the nominating season in the best financial position. He raised a staggering $25.1 million in January -- and, despite spending more than he took in, still had $16.8 million in cash reserves before the opening contests of 2020.

By comparison, former Vice President Joe Biden started February with $7.1 million remaining in his campaign accounts, followed by Buttigieg at $6.6 million.

In an email to supporters, Buttigieg said his campaign had collected $11 million this month after strong finishes in Iowa and New Hampshire but it wasn't enough to meet the challenges ahead. "The goal posts have changed," the former South Bend, Indiana, mayor wrote in an email obtained by CNN.

"We are no longer just up against Bernie Sanders, the Washington politician who has been running for president for years. We are now also up against a billionaire who is throwing colossal sums of money on television instead of doing the work of campaigning," Buttigieg said in the email, which was first reported by Politico

Sen. Amy Klobuchar of Minnesota raised a little less than Buttigieg in January, $5.5 million, and burned through $7.6 million that month, the new filings show. That left her with $2.9 million remaining in her campaign war chest.

But Warren opened February in the most precarious financial position of any top-tier candidate in the Democratic field. She had the smallest cash reserves: just $2.3 million, after spending $2 for every $1 she raised in January.

Her campaign used the line of credit it secured to set aside $400,000 to guard against running out of money before the Iowa caucuses. Campaign spokesman Chris Hayden said Warren did not have to use borrowed funds in the end.

And her aides said the senator's commanding performance during Wednesday's debate in Las Vegas had paid off -- driving $5 million into her campaign coffers in the day after the debate and attracting legions of first-time donors.

So far in February, her campaign has raised $17 million -- its best monthly financial showing to date, Warren officials said.

Thursday's filings also underscore the lopsided spending by the billionaires seeking the Democratic nomination. Bloomberg, who has skipped the first four contests to focus on the delegate-rich Super Tuesday states, is in a class of his own after donating an unprecedented $464 million of his media and financial-data fortune to his campaign. Meanwhile, former hedge fund manager Tom Steyer plowed more than $267 million of his own funds into his campaign through the end of January.

One sign of the the changing financial terrain in 2020: Warren -- who made the rejection of big money a cornerstone of her presidential bid -- on Thursday indicated she would not disavow help from Persist PAC , a new group that cropped up this week to boost her prospects.

Speaking to reporters, Warren said that all the men remaining in the race "had either super PACS or they were multi-billionaires, and could just, you know, rummage around in their sock drawers and find enough money to be able to fund a campaign."

"The only people who didn't have them were the two women," she added. "And at that point, there are some women saying, 'You know, that's just not right.' So here's where I stand: If all the candidates want to get rid of super PACS, count me in, I'll lead the charge. But that's how it has to be. It can't be the case that a bunch of people keep them and only one or two don't."

The super PAC, which can raise unlimited funds from virtually any source but must operate independently of Warren, is spending nearly $800,000 on ads in Nevada, according to a tally by Kantar Media's Campaign Media Analysis Group.

Another super PAC has emerged to aid Klobuchar. Dubbed Kitchen Table Conversations, the pro-Klobuchar group has spent $1.1 million on ads in Nevada, which holds its caucuses Saturday, and the South Carolina primary a week later.

Klobuchar's campaign officials said in October that they didn't want help from a super PAC. An aide said Wednesday that they stand by that position.

The groups won't have to disclose their donors until mid-March -- after Democrats in more than two dozen states have already weighed in. However, EMILY's List, a group that works to elect female candidates who support abortion rights, has donated $250,000 to each super PAC, EMILY's List spokeswoman Christina Reynolds said.