MINNEAPOLIS (CNN) -- Outspoken comedian and liberal radio talk show host Al Franken has told associates he plans to challenge Sen. Norm Coleman, R-Minn., in 2008, a source close to Franken said Wednesday.

Franken, 55, a Democrat who grew up in the Twin Cities and moved back to the state from New York last year, announced Monday that he was leaving his radio show on the Air America network after his Feb. 14 broadcast.

Franken has said previously that he was considering a bid to unseat Coleman, a moderate Republican and former St. Paul mayor who was elected to the Senate in 2002. During the 2006 midterm election, Franken's political action committee, the Midwest Values PAC, raised money for Democratic candidates across the North Star State.

Franken has been a writer and performer on the NBC program "Saturday Night Live." He is also the author of several politically tinged books, including "Rush Limbaugh is a Big Fat Idiot and Other Observations."