overease

I'm confused about the way the potential filibuster of the Alito nomination is being discussed, as in this article about Barack Obama's lack of enthusiasm for it. What I don't understand is why supporters of a filibuster need forty votes. Isn't it, rather, the case that opponents of the filibuster need sixty votes? The difference is non-negligible: you don't have to participate in a filibuster to not vote against it. Shouldn't we be asking those Democrats who are voting against Alito, but say they don't support a filibuster, just to sit out the vote to stop it?