WASHINGTON (REUTERS) - Democratic candidates running for the US Senate in nine hotly contested races amassed more than US$240 million (S$326.2 million) in contributions during the third quarter, leaving their Republican opponents far behind in fundraising as the campaign entered its final weeks.

But as candidates approached a midnight filing deadline on Thursday (Oct 15), Federal Election Commission records and campaign announcements also showed some Republicans only marginally behind in cash at the end of the quarter, with a few ahead.

Republicans are battling to keep their 53-47 majority in the Nov 3 election. Nonpartisan election analysts rate 12 Senate races as competitive this year, with 10 Republican-held seats and two Democratic-held seats in play.

Financial firepower will help determine whether Republicans fend off a powerful drive by Democrats to take the control of the Senate and set the legislative agenda in Congress next year.

Democrat Jaime Harrison, who is running neck and neck against Republican Senator Lindsey Graham in South Carolina, raised a staggering US$57.9 million during the three-month period ended Sept 30, more than twice the US$28 million reported by Graham's campaign.

Democrat Mark Kelly collected nearly US$39 million, besting a robust US$23 million fundraising total for his Arizona opponent, Republican Senator Martha McSally. Kelly entered October with nearly US$19 million in cash, vs. McSally's US$12.2 million.

In the key swing state of North Carolina, Democrat Cal Cunningham raised US$28.3 million in the quarter, dwarfing Republican Senator Thom Tillis' US$6.6. million.

Montana Democratic Governor Steve Bullock raised US$26.9 million, vs. US$11.5 million for Republican Senator Steve Daines.

But Daines ended the quarter with US$3.5 million in cash on hand, compared with Bullock's US$2 million.

Not all candidates have reported their fundraising totals.