Not an opportunity to gain ground--no one expects John Paul Stevens' replacement to fundamentally alter the ideological makeup of the court. However, the confirmation fight could get heated and ugly if Teh One decides to go all out and nominate a hard-left radical.
Senate conservatives could use this to make the hay fly, adding the nomination of a radical to a long and growing list of hard left, out-of-touch transgressions. Overly liberal, far-reaching federal courts are favored targets of conservatives and the argument has traction with independents who don't much care for radical swings in either direction.
Unless Obama nominates a Center Wing Nut Job and bona fide moderate, Conservatives should try to paint the prospective nominee as another Goodwin Liu, or perhaps Obama's version of Robert Bork. Let the fight drag through the summer, let it get as ugly as it needs to be, and let's see how it all shakes out in November.