Ed Kilgore at TPMCafe points out that Bush is in a tight spot with both Rehnquist and O'Connor retiring at once, since the religious right is unlikely to accept a compromise on either justice. Both justices must be sufficiently pro-life to support the overturn of Roe v. Wade. In fact, according to Kilgore's math, the next three nominees must take this stance:
Right now the Court is stacked 6-3 against overturning Roe v. Wade, based on the lineup in the 1992 Casey decision. That means overturning Roe--the obsessive and irreplaceable goal of the Cultural Right, not just with respect to the Supreme Court, but in terms of its alliance with the GOP--requires a net gain of two Justices for that position. Rehnquist actually voted against the original Roe decision. So his retirement would leave just two sure votes to overturn.
Thus, the Right has to run the table--a vote against Roe to replace O'Connor, a vote against Roe to replace Rehnquist, and a vote against Roe to replace the next retiree, probably Stevens.
As Kilgore puts it:
[W]hen it comes to the Court, the Cultural Right could not possibly care less about issues like business regulation or federalism or treatment of detainees at Gitmo. It's all about abortion--always has been, always will be.
If the Dems don't completely botch this--and with Reid at the helm, we can hope that they won't--Bush will be under pressure to nominate a moderate for at least one spot. If he had the benefit of several months between nominations, he'd arguably have to compromise less. Bush's early calls to tone down the heated rhetoric surrounding the first anticipated nomination are probably not so much directed at the left as at his own base.
According to this data, a slim majority of Americans supports Roe, and while most (78%) think abortion should remain legal in some circumstances, large majorities think it should be confined to the first trimester. Leaving abortion aside for a moment, a justice who would vote to overturn Roe would likely take a hard right position on any number of other issues. Who is appointed to the high court is arguably the most important consequence of last year's election. While Bush's supporters can point to his victory in November to claim that he should be able to appoint who he wants ("you lost, so quit whining and deal with the consequences"), the relatively close margin of victory, Bush's current abysmal poll numbers, and public opinion on Roe (among other issues) indicate otherwise.
I'd like to think the Dems could manage to pull this off. It depends on whether they can keep up the unity and focus they've displayed since the 2004 election, now when it matters most.