One thing I've noticed about the Left over the years is that they have trouble facing reality as it is. When someone, like yours truly, disagrees with the Left on any point, it threatens their version of reality, which makes them defensive. Then, they pull out anything they can from their bag of Alinsky-inspired tricks.
Take the TEA Parties. When they started last year, the Left ignored them at first. As they grew in popularity, the Left brought out their big guns and started mocking them. Now that many more Americans believe in the basic principles of the TEA Parties, the Left are resorting to slander, calling them racist and anti-government. Some on the left have gone so far as to compare the TEA Parties to the Ku Klux Klan and Timothy McVeigh. Of course, the truth is few, if any, arrests have been made at TEA Parties and the rhetoric that you'll find at these events are anything but racist and anti-government. (And I'm sure my Leftist counterpart will go out of his way to post pictures "proving" me "wrong," but there's a vast difference between being an advocate of smaller government and being anti-government.)
The TEA Parties aren't the only ones getting slimed by the Leftists. Financial institutions (Goldman Sachs), insurance companies (AIG), mortgage companies, Sarah Palin, Rush Limbaugh, Glenn Beck, and countless others have been slandered and libeled by the Leftists, all because they disagree with the President. (Of course, when it was the Left disagreeing with George W. Bush, it was patriotic, according to Hillary Clinton and the Left...) The Left may think it's "speaking truth to power" by attacking Republicans and conservatives dishonestly, but I think there's another reason.
They're scared of losing everything they've built in the past few years.
Back in 2006, the Left used a series of carefully crafted lies to get back into power in Congress. Then, they went along and did everything they chastised Republicans for doing, but more of it. Then, in 2008, the Left ran a candidate who ripped everything George W. Bush did...and then copied a lot of it once he got into the Oval Office. Their political ascension of recent years has been based on lies, and people are starting to realize it. The more people who see through the Left's lies, the fewer people the Left can keep on their side. That makes them desperate.
That's when the Left turns to slander and libel to diminish the impact that their opponents have. In other words, the Left tries to protect its lies by lying even more. Yeah, that always works.