* This presidential campaign costliest ever
* Romney, Republican Party amass cash as vote nears
* Adelsons give $10 million more to pro-Romney Super PAC
Mitt Romney and his Republican allies raised more than President Barack Obama and the Democrats in the first 17 days of October as both presidential campaign efforts approached the $1 billion mark, with Obama in the lead, disclosures showed on Thursday.
The tight 2012 presidential race has become the costliest in U.S. history thanks to massive spending on advertising and get-out-the-vote efforts by the campaigns and by outside groups that have no fundraising limits.
After several months of lagging Obama in fundraising, Romney, the Republican National Committee and allied state parties overtook their Democratic rivals in October with $111.8 million raised from Oct. 1 to 17, his campaign said, a period that included two of the three televised presidential debates.
Obama's campaign raised $90.5 million for itself and Democratic Party allies during that period, becoming the first in history to raise a total of more than $1 billion for an election effort together with his party.
Romney and the RNC were close behind, taking in a total of $919.4 million throughout the 2012 campaign so far.
Obama's campaign hedged on the $1 billion figure, saying the total was still short of that mark - at about $988 million - based on a tally of the cash raised since April 4, 2011, when Obama's team and the party officially became one campaign.
Reuters totals are based on a more commonly used calculation method that includes campaign funds starting on April 1, 2011, and party efforts dating back to Jan. 1, 2011.
"As the Romney campaign and their super PAC allies continue to outspend us on the air, we're making every effort to expand our donor base heading into the final stretch," said Obama campaign spokesman Adam Fetcher.
Romney's campaign said it had $169 million left in its own and the party's bank accounts at the end of Oct. 17 - a hefty sum for use in the last leg of campaigning before the Nov. 6 election and more than the $123.9 million available to Obama and the Democrats.
"There are less than two weeks left, but we still have much hard work to do to ensure that Mitt Romney and (running mate) Paul Ryan win in November and bring real change to Washington," said Spencer Zwick, Romney's national financial chairman.
With the two men running neck and neck in polls, Thursday's money disclosures are the last glimpse into the campaigns' finances before Election Day.
"In an election this close, they're certainly going to try to put every dollar to use," said Anthony Corrado, campaign finance expert and government professor at Colby College in Maine. "They're building up cash because they want to have as much money as they can for the big final push."
Obama voted early in Chicago on Thursday to encourage other Democrats to do so as both campaigns were in a hectic last-minute dash through the handful of crucial states seen as determining this year's winner.
WAR OF THE AIRWAVES
In a record-breaking spending spree, both campaigns have devoted the bulk of their cash to blanketing the airwaves with advertising.
In the first three weeks of October, the latest tally by the academic Wesleyan Media Project found that Republicans - Romney, his party and outside "Super PAC" groups - spent more than Democrats on ads, but Obama and his allies actually ran more ads.
Only candidates themselves are promised the cheapest rates for ads, meaning parties and outside groups have to pay up. While Romney has held a cash-on-hand advantage over Obama in recent months, it is thanks to strong fundraising by the Republican Party. Obama's cam p aign on its own has dwarfed Romney's in the money stakes in recent months.
That has afforded special prominence to the deep-pocketed Republican "Super PACs," outside spending groups that can raise unlimited funds and buy ads on Romney's behalf as long as their work is formally uncoordinated with the campaign.
The main pro-Romney Super PAC Restore Our Future on Thursday reported raising $20.1 million in the first 17 days of October.
Of that, $10 million was a new infusion from billionaire Las Vegas casino mogul Sheldon Adelson and wife Miriam, financial disclosures showed on Thursday. Harold Simmons, who is a Dallas banker and another top donor, gave $1 million.
The "super" political action committee had $24.2 million sitting in the bank as of Oct. 17, the filing showed.
The pro-Obama Super PAC Priorities USA Action raised $12.7 million and had $10.1 million left in the bank on Oct. 17.
The group received last-minute help from Silicon Valley start-ups, getting $1 million donations from LinkedIn co-founder Reid Garrett Hoffman, Zynga CEO Mark Pincus and Silicon Valley venture capitalist Vinod Khosla. Prominent billionaire financier George Soros also gave $1 million.
Another strong outside money group on Romney's side is American Crossroads, a Super PAC that is part of the effort spearheaded by veteran Republican operative Karl Rove, a former top aide to George W. Bush.
The group brought in $11.7 million in the first 2-1/2 weeks of October and had $6.4 million in cash on hand at the end.
After months of staying on the campaign sidelines, natural gas advocate and billionaire CEO of BP Capital T. Boone Pickens gave his first $1 million to the group earlier this month.
Other notable October contributions came from top Republican givers and the group's perennial donors Simmons, who gave $4 million, and Texas home builder Bob Perry, who gave $1 million.