10/08/2015 18:58 EDT | Updated 10/08/2016 01:12 EDT

AP source: Republican presidential candidate Marco Rubio raises $6M over last 3 months

WASHINGTON — Marco Rubio has raised roughly $6 million over the last three months for his Republican presidential bid.

That's according to an attendee at the Florida senator's donor retreat in Las Vegas, who spoke on the condition of anonymity to share fundraising numbers before they're officially released by the campaign.

Rubio's poll numbers may be rising, but the new fundraising total is far less than some of his rivals for the quarter that ended Sept. 30. It reflects what those briefed at the donor retreat describe as a difficult fundraising environment over the summer.

By comparison, retired neurosurgeon Ben Carson raised more than $20 million over the same time period.

The quarterly figures mark a practical and symbolic measure of strength for all presidential campaigns, but they don't tell the whole story.

Rubio, like most of the 2016 presidential field, also benefits from an allied super PAC and non-profit group that can raise and spend unlimited sums of money on his behalf. He's not legally allowed to co-ordinate with those groups, however.

Combined with money raised earlier in the year, Rubio's campaign started October with $14 million cash on hand, according to Thursday's donor briefing. That's more than Carson had in the bank after expenses, suggesting that Rubio's team is spending its resources more cautiously than some of his competitors.

Overall, the Rubio campaign told donors Thursday that fundraising improved in September, when it raised $1 million in online donations alone. October, which wasn't included in the quarterly total, will be the campaign's best month so far, according to the briefing.

Steve Peoples, The Associated Press