Another possible objection, for Democrats, is that she would have to recuse herself from all cases in which, as solicitor-general, she played a role. That could be a huge number of cases. Ed Whelan, a socially conservative court-watcher, supplies some historical context:
[L]et’s look to the last justice who was appointed to the Court from the position of Solicitor General, Thurgood Marshall.
Marshall served as Solicitor General from August 1965 to August 1967. He joined the Court on August 31, 1967. According to Lawrence S. Wrightsman’s The Psychology of the Supreme Court (p. 79), “Marshall recused himself from 98 of the 171 cases that were decided by the Supreme Court during the 1967-1968 term.” That’s 57% of the total. (Wrightsman states that “most of these were cases in which the federal government had been a party”; I suspect that all or nearly all of them were.)
One well-informed source tells me that the percentage of cases in which the United States takes part is much higher than in Marshall’s day and that in a recent term that figure approached 80%. My quick tally of the hearing lists for cases argued so far this term yields a figure of around 76%.
In other words, during Ms Kagan's first year, the court would have a 5-3 conservative-liberal split on most cases, instead of 5-4. That's something to think about.
Here's my question: if she's the most popular choice among conservatives--if she's best buds with Scalia and gets the endorsement of Bill Kristol--doesn't it make sense to keep her in reserve in case one of the more conservative justices retires? Then you'd be moving the court leftward overall in such an event.