Character assassination isn't the only way many on the left seek to gain power. As we saw in the debate in Wisconsin over budget reform, they're not above physical intimidation, and, of course, there's always vote fraud.
Perhaps such tactics are not confined to the left (although I'm really not aware of an instance where lies, threats, or fraud have been employed in the service of conservative ideas), and certainly not all liberals endorse such methods. Williams, after all, is a liberal.
The problem is that when politics becomes one's god, as it has for many on the secular left, then the end of political success justifies whatever means it takes to achieve it. If character assassination, threats, fraud, and dissimulation work in securing political power, well, then, they're morally appropriate tactics, and not only is there no reason not to employ them, it's actually one's moral duty to employ them.