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“Bumps in the road?” Mr. Romney asked incredulously at a rally at the Pueblo airport. “We had an ambassador assassinated. … These are not ‘bumps in the road,’ they’re human lives.”

Obama outspending Romney

Even though Mr. Romney has suffered some recent campaign missteps, a Quinnipiac Poll released last week showed Mr. Obama up by a margin of just 48 percent to 47 percent in the state, while a Rasmussen Reports survey issued the same day showed Mr. Romney leading by 47 percent to 45 percent.

A Purple Poll Strategies survey released Friday shows Mr. Obama leading by 3 percentage points in Colorado. All of the surveys show the candidates within the margin of error.

The Obama campaign has outspent the Romney camp by a margin of three to one in the Denver media market, according to a study by the University of Colorado News Corps. Obama for America was the largest spender in the Denver market, with $4.6 million in ad buys through Nov. 6, compared with just $1.4 million by Romney Victory for ads through last week.

Among so-called super PACs, Priorities USA, a pro-Obama group, had spent $1.9 million, while the conservative American Crossroads had purchased $1.2 million in advertising, said the study, released Sept. 16.

If not for the Obama campaign’s advertising advantage, “I think Romney could have been ahead by now,” said Mr. Ciruli. “And those ads — not only are they three or four to one in favor of Obama, but they’re relentlessly negative. And the unrelenting negativity has hurt Romney with his favorability [rating].”