* McCain attacks Obama on taxes

* Obama drops off trail to visit ill grandmother

* McCain behind in several key states

By Steve Holland

DURANGO, Colo., Oct 24 (Reuters) - Republican John McCain, fighting for traction 11 days before the U.S. presidential election, said on Friday he has a tough battle against Democrat Barack Obama in Colorado but vowed to make a comeback.

Down in opinion polls, the Arizona senator faces an uphill battle in several battleground states won by Republicans in 2004 but he and his aides say victory is still possible despite strong political winds blowing against him.

"This is going to be a tough state, my friends, and we're going to be up late (on election night) but we're going to win here," McCain told a noisy crowd in Denver.

"We need your vote, we need you to work. It's 11 days," McCain told a large crowd at a later rally at a high school football field in Durango.

McCain's predicament was summed up by former Denver Broncos quarterback John Elway, famed for leading late-game comebacks in the National Football League, who helped introduce him in Denver.