More than 30 elected prosecutors around the country have signed legal papers supporting a new trial for a Missouri man who has been in prison since his 1995 conviction for first-degree murder.

Now the case is headed for the Missouri Supreme Court in a legal effort that could bolster or undermine efforts by prosecutors nationwide to reopen cases where there are glaring questions about the verdict but unclear legal avenues for pursuing exoneration.

On Tuesday, a Missouri appeals court said it had no choice but to dismiss a case that sought to allow the elected prosecutor in St. Louis, Kim Gardner, to obtain a new trial for Lamar Johnson, whose case has become a cause célèbre for many recently elected district attorneys across the country.

Ms. Gardner is seeking the trial to present new evidence in hopes of finding Mr. Johnson innocent; in the years since his conviction, the only significant witness against him recanted, and two other men confessed that they alone were responsible for the crime. St. Louis prosecutors say the case was also marred by other perjury and prosecutorial misconduct.