A couple of old cases:
WASHINGTON, June 27 - The Supreme Court ruled on Monday
that the police did not have a constitutional duty to protect a person from harm, even a woman who had obtained a court-issued protective order against a violent husband making an arrest mandatory for a violation.
Supreme Court rules that police do not have constitutional duty to protect person from harm, even woman who obtained court-issued protective order against violent husband which made arrest mandatory for violation; decision overturns ruling by federal appeals court in Colorado; it had permitted...
www.nytimes.com
And Warren v. Columbia.
"The trial judges held that the police
were under no specific legal duty to provide protection to the individual plaintiffs and dismissed the complaints"
en.wikipedia.org
I'm not saying most cops wouldn't, but it's pretty well established unless you have a new case being ballied about.