LAS VEGAS -- Sen. Marco Rubio raised about $6 million for his presidential campaign in the third quarter of this year and entered the month of October with about $11 million on hand, campaign officials told top donors here Thursday.

Rubio's fundraising was slow through much of the summer, but ticked up considerably in September, and the campaign is on track to have its best fundraising month ever in October, campaign officials said at a donor summit in Las Vegas, according to a source in attendance.

The source, who requested anonymity because the totals had not yet been released publicly, said Rubio's campaign officials stressed that they had been budgeting accordingly and running a frugal operation. The officials told donors that Rubio had raised more than $1 million online in September alone, but described July and August as particularly difficult months because of the entry of rival candidates and the summer vacation season.

Because of his modest take over the summer, Rubio’s campaign was operating at a steep 81 percent burn rate – racing through nearly $4.6 million of the $6 million raised in the third quarter, according to a calculation by The Washington Post.

Rubio's totals fall far short of the leading Democratic candidates, former secretary of state Hillary Rodham Clinton and Sen. Bernie Sanders (I-Vt.), whose campaigns said they raised $28 million and $26 million respectively in the third quarter.

It is too soon to tell where Rubio will stack up in the vast Republican field. Few GOP campaigns have revealed their fundraising totals thus far; they are not required to file federal quarterly disclosures until Oct. 15. The campaign of Ben Carson, a retired neurosurgeon who has enjoyed grassroots momentum this summer, said it raised about $20 million, while Sen. Rand Paul's campaign said it had raised just $2 million.