Protesters say the zone violates the First Amendment to the U.S. Constitution.
The 2007 Massachusetts law makes it illegal, except for staff or clients, to "knowingly enter or remain on a public sidewalk adjacent to a reproductive health facility, within a buffer zone that extends outward 35 feet of any entrance, exit or driveway," NBC reported.
NBC said some justices questioned the size of the buffer zone, and a majority of the nine-member court seemed opposed to it.
SCOTUSBLOG.com said Chief Justice John Roberts remained silent throughout the argument, indicating he may be a key swing vote if any part of the law is to survive.
NPR said said the main plaintiff is Eleanor McCullen, a member of the anti-abortion group Operation Rescue and a part-time prison chaplain. McCullen has been standing for 13 years outside a Planned Parenthood clinic in Boston.
Law enforcement and clinic staff say the scene outside the facility is sometimes chaotic, NPR reported.
The last time the Supreme Court dealt with a similar case was in 2007, when a 6-3 majority upheld the constitutionality of a 100-foot zone in Colorado.
But four of the six-member majority have either retired or died. All three of the dissenters, Justices Anthony Kennedy, Antonin Scalia and Clarence Thomas, are on the present court and will vote in the Massachusetts case.
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