State Rep. Cory Gardner (R) is set to announce his plans to challenge freshman Rep. Betsy Markey (D) in the 4th district in 2010.

Gardner, who has served in the GOP leadership in Colorado’s General Assembly, was described recently by an National Republican Congressional Committee official as a rising star in Colorado GOP circles, and his state legislative district includes a large number of the Congressional district’s eastern counties.

Markey ousted Rep. Marilyn Musgrave (R) by a surprisingly large 12-point margin last November, but Republicans firmly believe the conservative 4th district, which had elected a Republican to Congress for 37 years before Markey’s victory, should still be in GOP hands.

An official announcement from Gardner could come as early as today.