* 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.

"Senator, it's the fourth quarter," Elway said. "Some pundits are already counting you out, but I know a thing or two about comebacks and I cannot wait until November 4 when you once again prove those pundits wrong and are elected as our next president of the United States."

McCain, as he has for much of the week, argued at events in Denver, Colorado Springs and Durango that Obama would raise taxes on small businesses. He again cited Obama's recent comment to an Ohio man, Joe Wurzelbacher, also known as "Joe the Plumber," that he wanted to "spread the wealth around."

"This will be a focal point in the last 11 days of this campaign," McCain told reporters in Colorado Springs after meeting small business owners at a steel fabrication plant.

Obama, in Hawaii to visit his seriously ill grandmother, told ABC's "Good Morning America" that he did not regret the comment that has given McCain hope of piercing Obama's armor.

HIGH-PROFILE DEFECTORS

"Not at all," the Illinois senator said. "For us to want to continue to give tax breaks to big corporations and the wealthiest, instead of the middle class that are desperately in need of some help right now, would not only be bad for the families, it would be bad as the economy as a whole."

Obama aides said in a conference call with reporters that they felt good about Obama's chances in battleground states like Colorado and Virginia as well as in Iowa and New Mexico.

"We assume that there's going to be some tightening (in the polls), and we assume that some of these states are going to be decided narrowly. So that's the kind of operation we've built in these states, is an operation that could help us win a very close election," said Obama campaign manager David Plouffe.

McCain's challenge emerges from polling numbers in states that in the past have been Republican strongholds. He is down about 5 percentage points in Colorado, down about 6 in Ohio, a couple in Florida, 3 in Nevada and Missouri, and 2 in North Carolina, according to an average of polls in these states compiled by RealClearPolitics.com.

Some high-profile Republicans have defected to Obama's side, including former Secretary of State Colin Powell, former White House spokesman Scott McClellan, both of whom served under President George W. Bush, and former Massachusetts Gov. William Weld.

McCain's aides say he is gaining some steam with his "Joe the Plumber" assault on Obama's tax plan and that he will continue the theme for several more days.

Looking out at the crowd in Durango, McCain saw signs of support for his Joe the Plumber theme.

"Joe the Florist is here," he said. "Joe the Carpenter. Dan the Rancher. Hi Dan. Another Joe the Plumber."

Traveling with McCain was close friend Republican Sen. Lindsey Graham of South Carolina, who tweaked Obama for his eloquence.

"And you know know what America needs? We don't need somebody who sounds good, we need somebody who works hard," Graham said. (Editing by Eric Beech)

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