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Why Colorado? Analysts say the avalanche of campaign spending has largely been prompted by close races and a Supreme Court ruling earlier this year that loosened campaign restrictions on businesses, labor unions and independent groups.

“Corporations are now free to spend unlimited amounts of money on campaigning,” said Paul Ryan, an elections law expert with the Campaign Legal Center, one of the groups that filed a complaint against Crossroads GPS. “It’s a green light to get involved in a politics and spend whatever you want.”

Colorado is also an attractive state for outside spending because the state has just about 5 million people, some 70 percent of them reachable by advertisements on Denver TV stations. That means a dollar spent in Colorado has more impact than a dollar spent in states with multiple big-city markets.

Mr. Bennet has been questioning the outside groups financing attack ads against him.

“It’s unprecedented in Colorado,” Mr. Bennet said. “Colorado’s become ground zero for Karl Rove and [conservative South Carolina Sen.] Jim DeMint to play with the voters. I think Colorado voters want this to be about Colorado.”

Mr. Buck, the beneficiary of so much outside spending, shrugged off suggestions that the Bennet-bashing coming from out of state will make the difference.

“I think what voters care about is the authenticity and the general message coming from the candidate and not all these outside groups,” he said.