For years, groups have applied to fly their flag outside Boston City Hall and have been approved, but the city refused to display a Christian flag and was sued — a lawsuit Boston is likely to lose, legal experts told Jim Braude on Greater Boston Tuesday.

"Boston basically made a mistake. It would not violate the establishment clause to put up this flag for one hour, on one day, in a program that had shown 284 other flags," said David Cole, national legal director of the American Civil Liberties Union.

The ACLU is is supporting Camp Constitution, the organization that applied to fly its Christian flag and was denied. The case was escalated all the way up to the Supreme Court, and justices heard arguments on Tuesday.

Rachel Laser, president and CEO of Americans United for Separation of Church & State, agreed with Cole, saying, "I think the court is likely to designate [the flagpole] a public forum, in which case even the City of Boston wouldn't argue that the Christian flag can't fly."

However, she added, "I would say that a chill wind blows for the separation of church and state."

Watch: The Supreme Court will weigh in on Boston's flagpole dispute: