To do that without a warrant violates the Fourth Amendment. The same, he suggested, might hold true for other new sources of information, such as outdoor video cameras and automatic toll equipment. Kagan agreed with all this.
Sotomayor took an even warier view of police use of modern data-collection systems. Under past Supreme Court decisions, you can unwittingly surrender your privacy by doing business with a bank, insurer or other company. The government can commandeer those records without a warrant -- on the odd theory that they are not private because you've already let someone see them.
Of course, the fact that you have to contract with a cell phone provider to function in the modern world doesn't mean you have no stake in keeping your call log strictly between you and Verizon. Sotomayor said the existing, government-enabling doctrine "is ill suited to the digital age." Her position, if shared by other justices, could lead to sensible new constraints on cops.
The Obama justices also firmly rebuked the government when it trampled on freedom of religion. The administration had taken the side of a religion teacher at a religious school who claimed she had suffered employment discrimination.
Ministers and other religious leaders are normally not covered by such laws, on the theory that the government has no business telling sectarian bodies who should lead the faithful. But the Justice Department not only said the teacher was not covered by the "ministerial exception"; it said there should be no such exception.
How did that argument work out? During oral arguments, Kagan called it "amazing," and the court rejected it 9-0. The religion clauses of the First Amendment, it said, "bar the government from interfering with the decision of a religious group to fire one of its ministers."
Obama would like to extend the government's reach into that as well as other places that were once off-limits. When he tries, though, he can't assume his justices will have his back.