Among critics of the federal prohibition of marijuana — a diverse and bipartisan group that includes both criminal justice reform advocates and Big Alcohol — the American Legion and its allies stand out.

For more than a year, the stalwart veterans group has been working to reframe the debate as a question of not only moral and economic imperatives, but also patriotic ones, arguing that access to medical marijuana could ease suffering and reduce suicide rates among soldiers who return from the horrors of war.

“We’ve got young men and women with PTSD and traumatic brain injuries coming to us and saying that cannabis works,” Joe Plenzler, a spokesman for the group, which was established after World War I and has over two million members, said by telephone Wednesday.

Mr. Plenzler said that veterans had turned to medical marijuana as an alternative to so-called “zombie drugs,” including opioids and antidepressants, that they said adversely affected their mood and personality, up to and including thoughts of suicide. In studies, cannabis has been shown to help alleviate chronic pain and reduce muscle spasms in multiple sclerosis patients.