FACTBOX: Weather Underground resurfaces in U.S. politics

(Reuters) - Sen. Barack Obama was asked during a televised debate about his links to a former member of the Vietnam-era militant Weather Underground organization, Bill Ayers, who is now a professor of education at the University of Illinois at Chicago.

The Illinois senator running for the Democratic nomination for president served with Ayers on the board of an anti-poverty foundation in Chicago.

Obama said he was only eight-years-old when the Weather Underground committed its best known bombing and was being falsely linked with "detestable acts".

He noted that Bill Clinton, husband of his opponent Hillary Clinton, pardoned two members of the group during the last days of his presidency.

Following are some facts about the Weather Underground:

- The Weather Underground was formed in 1969 originally as Weatherman or the Weathermen, emerging from radical left-wing and militant student protest groups opposed to the Vietnam War.

- The name was based on a line from a Bob Dylan song "Subterranean Homesick Blues" that ran: "You don't need a weatherman to know which way the wind blows."

- From 1969 until 1975 the group carried out a domestic campaign that included bombing the Pentagon, the U.S. Capitol, police facilities and banks. At least one police officer died and several injured.

- In March, 1970, the group planned to bomb Fort Dix Army Base in New Jersey but the bomb exploded prematurely at a townhouse in Greenwich Village, New York, killing three members.

- Some of its members are now academics, including Bernardine Dohrn and Ayers, who are married. Both were indicted in 1970 but charges were dropped later because of prosecutorial misconduct. In all, they spent around 10 years in hiding.

- The group dissolved in the mid-1970s with the end of the war and decline of leftist U.S. politics, though some members moved onto other radical groups.

- In 1981, Weathermen and members of the militant group the Black Liberation Army robbed an armored car in New York state, killing two police officers and a guard in a shootout before being arrested.

- Critics call the Weathermen domestic terrorists and oppose the rehabilitation of its members. The Weathermen's activities have been referenced in books, songs and TV shows including "American Pastoral" by Philip Roth.

(Writing by Matthew Bigg; editing by Tom Brown and Philip Barbara)