* Mexican peso slumps more than 7 pct vs U.S. dollar
* Futures down: Dow 2.5 pct, S&P 3 pct, Nasdaq 2.9 pct (Recasts lead with bets rising on Trump win; updates prices; adds comment)
By Rodrigo Campos
NEW YORK, Nov 8 (Reuters) - U.S. stock index futures tumbled in choppy trade on Tuesday as tight races in key states including Florida and Ohio stoked bets that Republican Donald Trump could win the U.S. presidential election, spooking investors who have been counting on a victory by Democrat Hillary Clinton.
Trump and Clinton were locked in close battles in several battleground states, although opinion polls gave Clinton an edge in the closing hours of the campaign.
Financial markets reacted violently to the preliminary results, with S&P futures rising as much as 0.8 percent and falling more than 3 percent at one point.
A risk-off mood pervaded financial markets, driving down stock index futures and the Mexican peso, while gold and Treasuries prices rose.
"Donald Trump has stunned the consensus thinking, thus far," said Peter Kenny, senior market strategist at Global Markets Advisory Group in New York.
"Ahead in Florida, Virginia, Ohio and North Carolina, in addition to being ahead in the electoral count and popular vote has triggered the onset of panic in global financial markets," said Kenny. "Too early to call, obviously, but clearly markets were tentative today and seem to being growing increasingly concerned.."
Wall Street sees former Secretary of State Clinton as a status quo candidate who would lend stability to the markets, while Trump's stances on foreign policy, trade and immigration are more conducive to volatility.
At 9:43 p.m. EST (0243 GMT), S&P 500 e-minis were down 65 points, or 3.04 percent, with 719,267 contracts changing hands. Nasdaq 100 e-minis were down 139.5 points, or 2.9 percent, on volume of 92,108 contracts, and Dow e-minis were down 458 points, or 2.5 percent, with 127,944 contracts changing hands.
The Mexican peso slumped versus the U.S. dollar to near historic lows hit in September. It was last down more than 7 percent, near 19.7 to the dollar, after earlier touching 18.1475, its strongest level since Aug. 6.
"The chance of a Republican sweep is now very real," said Brad McMillan, chief investment officer at Commonwealth Financial in Waltham, Massachusetts.
"With a Republican House and Senate, Trump now has a great deal more policy freedom - and it remains to be seen what he might do with it. Markets have reacted in the past with a decline as Trump's probability of victory rose, and that is very likely to happen (Wednesday) if he wins."
Additional reporting by Chuck Mikolajczak, Saqib Ahmed, Caroline Valetkevitch, Sinead Carew and Trevor Hunnicutt; Editing by Leslie Adler