I'm not naive enough to believe that only Republicans go negative in political campaigns. It's just that they seem dirtier somehow. While I'm sure there are exceptions, Democratic negative ads are at least tangentially related to truth. With Republican negative ads, all bets are off.
Here is a negative John McCain ad, made by a YouTube-er. This is what a real anti-McCain ad would look like if Democrats fought like Republicans do. And as far as I can tell, they didn't have to lie to do it.
McCain - Unfit To Lead (Draft version)
Found at: AmericaBlog