I quote others only to better express myself. -- Montaigne.
Scott Lemieux gets at the crux of the politics here:
The fact that conservatives are not openly touting Alito's conservative jurisprudence but rather trying to claim that his record doesn't mean what it obviously means tells you that conservatives themselves don't think that overturning Roe and many of the other long-held goals of conservatives are supported by the public. Democrats should not be running scared, and there is no reason to believe--pace Matt--that there is any kind of significant political downside if they strongly oppose Alito, which they clearly should on the merits.
I won't detail the "on the merits" comment -- see Lemieux's links for details (and see here for more). Of course, "the merits" mean the political merits -- unlike Harriet Miers, no one is claiming that Alito is unqualified. But those merits are perfectly valid reasons for rejecting a nominee. As Lemieux wrote about Robert Bork:
He was appointed for political reasons, of course, and rejected for political reasons, and in both cases the reaction was perfectly appropriate. It was Reagan's right to appoint somebody who believes that the Court's entire line of privacy cases is wrongly decided and that the federal government can legally segregate and that the 1st Amendment should be read extremely narrowly. And it was the Senate's prerogative to reject someone with these views.
It's nonsense to pretend that judicial nominations aren't political; so we shouldn't be ashamed or reticent about making explicitly political arguments and judgments about them. The Democrats should filibuster Alito because he is a right-wing judge who will make right-wing political judgments; it's the right political move, both in the sense of the moral thing to do and in the sense of the canny thing to do (if done properly).
Nor do any of the non-political arguments against filibustering stand up. The notion that judges shouldn't be filibustered is just silly -- the filibuster is more reasonable for judicial appointments, which are for life, than for political ones, which are time-limited, or political bills, which can be repealed. As for the notion that one shouldn't filibuster because it's counter-majoritarian, well, the Senate is counter-majoritarian; as Hendrik Hertzberg is wont to point out, "if each of every state’s two senators is taken to represent half that state’s population, then the Senate’s fifty-five Republicans represent 131 million people, while its forty-four Democrats represent 161 million". And -- the silliest argument of all -- the fact that the nuclear option might be invoked is not a reason not to use the filibuster: as Matt Yglesias has argued, the worst possible outcome is for Alito to get on the Court and for the filibuster to remain; ideally, Alito would be blocked, but if that can't happen, getting rid of the filibuster (which historically tends to help conservative causes more than liberal ones) would be a nice consolation prize.*
* Even if it is gotten rid of in an illegitimate way -- remember, the nuclear option is so-called not because it's getting rid of the filibuster, but because it is doing so in a way that breaks Senate rules. Still, breaking the rules is a bad thing -- so the best result would be a Democratic filibuster which the Republicans would simply fail to overcome.