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Justice Scalia, Justice Anthony M. Kennedy and Justice Clarence Thomas were viewed as likely to deem the law overbroad, given their dissent in the high court’s abortion-access case in 2000. Mr. Alito’s questions also appeared unsympathetic to the law.

If Chief Justice John G. Roberts Jr. joined those four in a majority, “buffer zones that are not confined explicitly to stopping violence or actual physical obstruction may well be doomed altogether,” court watcher Lyle Denniston wrote at SCOTUSblog.

Mr. Roberts and Mr. Thomas asked no questions; liberal Justices Breyer, Elena Kagan and Sonia Sotomayor actively participated.

In 2000, the Supreme Court voted 6-3 to uphold a buffer zone in Colorado in a decision that some free speech advocates, who also support abortion rights, have criticized. But four of the six justices in the majority on that decision are no longer on the court.

Outside the courthouse after the hearing, Ms. Coakley said the 2007 law strikes “a perfectly reasonable” balance and has kept the public peace. Liam Lowney, whose sister Shannon was killed at a Massachusetts clinic in 1994, said he hoped the court would uphold the law.

Eleanor McCullen, 77, said she offers love, hope and tangible assistance to pregnant women and, before 2007, she helped dozens of women change their minds about aborting their pregnancies.

But since the 2007 law was enacted, she has found herself greatly restricted from interacting with women entering the clinics. A yellow line is painted on the pavement to mark the buffer zone, and even if she is talking with someone, she said, “I stop at the yellow line.”