While Big Sky Country has been Republican at the presidential level for quite some time, at the state and local levels it has a strong Democratic streak extending back to the Progressive era. The western mountains are heavily unionized (Montana is one of the Mountain West states without a right-to-work law), and Democrats with a populist streak, like Gov. Brian Schweitzer, are popular in the state.
In 2006, Jon Tester defeated Sen. Conrad Burns. Burns is the only Republican who had ever been elected to successive terms in Montana history, despite his penchant for cringe-inducing gaffes (such as telling a group of firefighters, returning home from the field, that they had done a "piss-poor job"). Tester was a rancher and state senator who became a darling of the left-leaning Netroots in 2006, and ran a populist campaign against Burns. Despite the pro-Democratic tone of that cycle, Tester won only narrowly.
Tester seems to have grown into his role as Senator for the state, however. In 2012 he won by four points against Rep. Dennis Rehberg. This time he faces state Auditor Matt Rosendale. This state has swung toward Republicans as well, who control every statewide office with the exception of this seat and the governorship. But the national environment may insulate him against the state’s partisan tilt.