What effects will an Alito appointment have on Supreme Court doctrine? Fairly significant changes, particularly in the areas that social conservatives most care about.
Assume for the moment that Alito is confirmed by Christmas. The Supreme Court will probably hold over the New Hampshire abortion case, Ayotte, for reargument, if, as expected, the Justices vote 4-4 (excluding Justice O'Connor's vote, which will not count if she retires in December). On the other hand, if Justice Kennedy joins the liberals in Ayotte, Justice O'Connor's vote will not matter and the case will be decided 5-3.
What happens in Ayotte may be a harbinger of things to come. If successful, Alito's nomination will make Anthony Kennedy the median or swing Justice on the U.S. Supreme Court. In the past, Sandra Day O'Connor had held that powerful position, only occasionally displaced by Kennedy. Now it is largely Kennedy's, with occasional displacements by Breyer.
Put another way, to understand what Alito's appointment means for constitutional doctrine, instead of focusing on Alito's views (which one assumes are reliably conservative), one needs to focus on Kennedy's. We know that the new median Justice supports abortion rights claims a little less than O'Connor (Kennedy voted to uphold restrictions on partial birth abortion), supports gay rights claims a bit more than O'Connor (Kennedy wrote the opinion in Lawrence), thinks affirmative action is largely unconstitutional (Kennedy dissented in Grutter), thinks most campaign finance regulation is unconstitutional (Kennedy dissented (in part) in McConnell) and has been more likely to permit government endorsements of religion and state financial support for religion than O'Connor (Kennedy dissented in Mccreary County v. ACLU and joined Mitchell v. Helms). On federalism, it's a mixed bag: Kennedy joined Raich v. Ashcroft but dissented in the two most recent section five cases, Tennesee v. Lane and Hibbs. On Presidential power, the position of the new median justice, interestingly enough, appears to be unchanged. Although Rehnquist and O'Connor ... are gone, the Administration would still have lost Hamdi, because its position was opposed by Kennedy, Scalia, Stevens, Breyer, Souter, and Ginsburg.
Thus, Alito (together with Chief Justice Roberts) will cause some pretty significant shifts in Supreme Court doctrine, most notably in areas of religion, abortion, affirmative action, and campaign finance, but not yet in the area of gay rights. Changes in doctrines of federalism and presidential power are a bit less clear. These shifts will come in most of the places that movement conservatives are looking for changes in constitutional doctrine, which is why they will be delighted by an Alito appointment. The greatest irony, perhaps, is that although many movement conservatives now loathe Anthony Kennedy, all the changes in doctrine I've outlined will occur because Kennedy now holds the swing vote.
Finally, if Justice Stevens were to retire in the next few years, and be replaced by a staunch conservative, we would have a full scale constitutional revolution on our hands. For then the median Justices would be none other than John Roberts and Samuel Alito.