The reason the left is desperate to malign the tea party movement is simple: the left is threatened by it. Any cause that champions greater liberty and smaller government, more individualism and lower taxes, and accountability from elected representatives (like say...read a bill before you vote on it) is counter to what liberals want for this country.
We see this pattern across all issues:
If you support the tea party, you are a racist bigot, they say.
If you support the Second Amendment, you are a paramilitary, nutjob militia type, waiting to shoot up the shopping mall.
If you support welfare reform and job incentives, you hate the poor people.
If you support jailing captured terrorists, you are an enemy of civil rights.
Etc., etc., blah, blah...we all know the liberal line on all this stuff.
In actuality, it's the liberals who are the most oppressive people on earth. They thrive and gain power when people are dependent and powerless, and want to see us all reliant on a government that can never deliver on its promises.