Rep. Mark Udall (D-CO).

(CNN) – Democratic congressman Mark Udall has won the U.S. Senate seat in Colorado now held by retiring Republican Sen. Wayne Allard, CNN projects.

If perennially-embattled incumbent Allard hadn't retired, Udall — son of legendary Arizona politician Mo Udall — would still have held the advantage in this traditionally-Republican state. Udall's liberalism on social issues might have cost him in a typical cycle. But Democratic presidential nominee Barack Obama's popularity in the state, and the general anti-GOP climate, instead posed insurmountable obstacles for Republican Rep. Bob Schaffer.

Udall was painted as too liberal, and Schaffer too conservative, as both vied for the independent vote in a closely-fought contest. But as the campaign moved into the homestretch, Udall began to pull ahead — and Republicans wrote off Schaffer's chances weeks ago.