By Bill Mears, CNN

Boston (CNN) - Outside the Planned Parenthood Clinic in Boston on a recent winter day are the regulars - a small, devoted team of anti-abortion activists, handing out fliers and urging patrons to hear their message: "Save that child." "Every life is precious, protect that life within you." "Please change your mind." Several people pray silently nearby.

Clearly marked on the sidewalk, nearly 12 yards from the front doors, is a painted boundary, a line the protesters cannot cross. By state law, their First Amendment rights stop there.

A metaphoric line - testing the competing limits of what has become a constitutional fight between free speech and public safety - will now be surveyed by the nation's highest court.

The justices on Wednesday will step back into the larger national debate on abortion, when it holds oral arguments on a challenge to a Massachusetts law that established tighter buffer zones around facilities that perform the procedure.