The Supreme Court denied an effort to strip acting Attorney General Matthew Whitaker’s name from a pending case, swatting down a push to force the justices to send a signal that President Trump’s temporary pick is illegal. Lawyers had asked the justices to remove Mr. Whitaker and substitute Deputy Attorney General Rod Rosenstein, arguing that he should be the rightful head of the Justice Department under the law, after former Attorney General Jeff Sessions was ousted last year. The request came in a case involving Barry Michaels, who is challenging a law denying him the right to purchase a firearm...