“With the Trump candidacy, this is the first time we’ve sort of seen calls for censorship of, literally, support for a candidate,” Ms. Harris said. “For a long time there’s been a sense by students that they have a right not to be uncomfortable. You do have a right not to feel unsafe. The question is: What does unsafe mean?”

In other words, she said, students who write messages on the sidewalk in support of Mr. Trump would have a high bar to meet for their speech not to be protected by the Constitution. A message crosses into unprotected speech when it crosses into the type of harassment or incitement of violence that essentially bars a student from gaining access to an educational opportunity.

“One thing we see a lot of on campuses is the conflation of emotional and physical safety,” Ms. Harris said. “If I chalk ‘Trump 2016’ and someone says, ‘that makes me feel unsafe,’ that does not automatically convert it into a threat.”

Still, some school officials, like those at the University of Kansas, are moving to distance themselves from the messages, without calling into question the right of a student to write them.