I think the direction in which media swings depends on the political situation in the country.
In the late 1990s, television and radio seemed to swing right. Everyone was jumping over the Clinton impeachment hearings, because they wanted to see the first President impeached.
During the 2000 election, television went left, while radio stayed right. Even though I was a kid, I sort of knew this, because every media outlet on TV seemed to scrutinize Bush for his drinking, drug use and random swearing. Meanwhile, Al Gore acted like a pompous jerk during the debates, and the pundits declared him the winner despite his lack of tact.
Following 9/11, everything went right. From the terror alert to anthrax to the duct tape crisis, the media followed every waking minute of the 9/11 aftermath and virtually promoted the idea of going into Afghanistan. As far as I can remember, the media seemed to love the idea of going into Iraq, too. It was also during this time that Fox News became the number one cable news network.
In 2004, radio stayed right, TV started to lean left... but John Kerry seemed to gain more scrutiny than George W. Bush on a daily basis.
Of course, following the 2004 election, TV went left, while radio saw a bit of a shift (Air America debuted in 2004, but didn't become 'big' until 2005). Then again, Rush Limbaugh was still blabbering, making fun of Michael J. Fox and whatnot.
TV is currently left-leaning, and the radio is currently going back and forth.
I think it just depends on the current state of affairs, and that no one media stays biased towards one side.