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What the issue is: William Ayers is a University of Illinois, Chicago, professor who in 1969 cofounded the Weather Underground group and helped plant bombs, including one in 1971 at the U.S. Capitol. Barack Obama became acquainted with him in 1995.

Ayers turned himself in after spending the 1970s on the run. Federal prosecutors eventually dropped charges against him because of misconduct by law enforcement, including illegal surveillance. Ayers, 63, is a tenured professor of education who has been active in trying to improve the Chicago public schools. Chicago Mayor Richard Daley has defended Ayers against attacks.

In 1995, as Obama embarked on his first run for the Illinois state Senate, Ayers and his wife, Weather Underground cofounder Bernardine Dohrn, hosted a house party to introduce him to other Democrats. Ayers later gave Obama a $200 campaign donation. They also served together on two philanthropic boards.

Obama has described Ayers as a "guy" he knows from his Hyde Park neighborhood and someone with whom he served on boards, but not one of his advisers. Obama was a child when the Weather Underground was active.

Why the McCain campaign says it matters: Obama's decision to associate with Ayers raises questions about his judgment. McCain's running mate, Sarah Palin, has said that Obama is someone who would "pal around with terrorists," and that Obama "sees America as imperfect enough to work with a former domestic terrorist who targeted his own country."

Why the Obama campaign says it doesn't matter: Obama is not close to Ayers. McCain is focusing on Ayers "to cover up McCain's erratic response to the most urgent economic crisis facing our nation since the Great Depression," Obama spokesman Ben LaBolt said.

DAN MORAIN, LOS ANGELES TIMES