WASHINGTON - The selection of Solicitor General Elena Kagan to be the nation's 112th justice extends a quarter-century pattern in which Republican presidents generally install strong conservatives on the Supreme Court while Democratic presidents pick candidates who often disappoint their liberal base.

Ms. Kagan is certainly too liberal for conservatives, who quickly criticized her nomination on Monday as a radical threat. But much like every other Democratic nominee since the 1960s, she does not fit the profile sought by the left, which hungers for a full-throated counterweight to the court's conservative leader, Justice Antonin Scalia.