NEW YORK (CNN) -- Humorist and liberal radio talk show host Al Franken will make an announcement about his decision to run for U.S. Senate on Feb. 14, during his last radio broadcast on the Air America network, two sources with knowledge of his plans tell CNN.

Earlier this week, another source close to Franken told CNN that he planned to challenge Sen. Norm Coleman, R-Minnesota, in 2008. Franken, 55, a Democrat who grew up in Minnesota 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 former Democrat turned Republican 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."