LIBERALS ON THE TEEVEE…. Starting Sept. 8, MSNBC will feature two — count ’em, two — left-leaning voices in its primetime lineup. Plenty of people apparently don’t know what to make of these shocking developments.

With the promotion of Rachel Maddow, the Air America radio host, to a prime-time television spot this week, the longtime third-place cable news network MSNBC cemented its identity as a channel for a liberal audience.

But is that what advertisers want it to be?

MSNBC, which is owned by NBC Universal, a unit of General Electric, does not trumpet its shift. But in the 12 years that MSNBC has competed head-to-head with CNN and the Fox News Channel, the partisan lines have never been drawn so neatly.

I can appreciate that MSNBC’s lineup is outside the norm for American media, but asking whether advertisers are ready for the one-two punch of Olbermann and Maddow seems to miss the point. Advertisers turn to television shows that people watch.

If we turn back the clock a bit, we see that it wasn’t too long ago that MSNBC added Tucker Carlson to its primetime lineup, apparently hoping to capitalize on Fox News’ success appealing to a Republican audience. If cable-news viewers were flocking to a conservative, partisan network, MSNBC seemed to believe, then the answer was to keep up by putting more conservatives on the air.

The result was Fox News-lite — a lineup with conservative Carlson, conservative Joe Scarborough, and the politically baffling Chris Matthews, who, within a single program, is capable of both praising Obama and criticizing him for drinking orange juice. (At one point, MSNBC even gave Michael Savage a show.)

Viewers didn’t exactly flock to this lineup. There’s already a Republican news network; no one needed a pale imitation. But with Olbermann’s ratings continuing to blossom, MSNBC seems to think it’s time to offer something different. It strikes me as good business sense, and a healthy dose of common sense.

“Is that what advertisers want it to be?” If viewers show up, and I’m confident they will, the advertisers will be fine.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.