The flier from the Colorado Republican Committee says "Obama has close ties to domestic terrorist" William Ayers. The mailer has also appeared in Wisconsin and North Carolina.
Ayers founded the Weather Underground, a group that planted bombs at government buildings in the early 1970s.
Obama and Ayers, now a professor at the University of Illinois-Chicago, lived in the same Chicago neighborhood and served together on a charity's board. Ayers hosted a 1995 meet-and-greet event at his home when Obama was launching his first run for Illinois state Senate. Still, there is no evidence they are close friends.
Republicans have accused Obama of "palling around with terrorists." Obama has responded that he was 8 years old when Ayers "engaged in despicable acts with a radical domestic group."
The mailing comes as John McCain visited Durango on Friday. McCain has only occasionally referred to Ayers during the campaign.
The flier calls Ayers a "friend of Obama" and says the Democratic U.S. senator is "not who you think he is."
Dick Wadhams, Republican state chairman, defended the mailing, which he said was sent to targeted voters throughout the state this week.
"It's a legitimate issue," Wadhams said. "There's a pattern of behavior about Sen. Obama."
Wadhams mentioned Obama's ties to the Rev. Jeremiah Wright, the Chicago pastor whose fiery sermons have received wide attention, and Tony Rezko, a donor to past Obama campaigns who was convicted this summer on federal charges of squeezing kickbacks out of firms wanting to do business with the state.
Sen. Hillary Clinton, Obama's toughest primary opponent, was the first to question Obama's relationships, Wadhams said.
"Why would it not be legitimate now if it was OK for Sen. Clinton to bring it up during the Democratic primary?" he asked.
Matt Chandler, Colorado spokesman for the Obama campaign, said the mailing was part of "scare tactics to try to change the subject."
"Voters across the country, including Republicans like Colin Powell, are rejecting these kinds of false attacks because they are a sad and desperate attempt to distract from the fact that John McCain is more of the same," Chandler said.
chuck@durangoherald.com.
The Associated Press contributed to this report.