If in the course of Alito's confirmation hearings it is shown that he is indeed an ideologue whose judicial views are of an extreme nature, his nomination must be opposed. Many indications are that Alito's extremism and ideology might well make Scalia look like Sandra Day O'Connor. The democrats and the left will only be making harder for the Senate to oppose Alito's nomination if they continue to compare him to Scalia.
It would be far more instructive (and far more constructive for that matter) to contrast Alito to Scalia. If opposing Alito is to be justified (and supported by the American people), it will be incredibly difficult to do so unless he is shown to be different from Scalia.