Butlers political career died with the Progressives during the 1930s. He missed getting on the New Dealers train, in part because he disliked Roosevelt. Butler was simply a democrat (small d) and old school progressive. He'd have continued on the periphery of politics through the 1940s. Post 1945 he might have gained considerable traction if he had focused on veterans affairs, tho he had earlier in the 1930s damaged himself through his criticism of the American Legion leadership. He had also missed the bus within the USMC in the 1920s by opposing the development of amphibious warfare techniques. While on active service in the 1920s he had criticized the "drones" of Quantico, junior officers who were experimenting with amphibious warfare techniques. He seemed to think they'd be better employed chasing Sandanistas or Hatian bandits around Latin America. That ran counter to USN plans and ended any small chance of a shot at Commadancy of the Marine Corps.
The guy was brilliant & a great leader. Well schooled in tactics and operational planning & execution. But his education in the upper levels of politics, strategy, world affairs, ect... was uneven with some large holes. As police commissioner in Philidelphia and later with the Oregon State law enforcement he proved a asset, until he misread local politics and was maneuvered out the door.
... Long story short, it wasn't an actual thing. There was absolutely no chance of getting Smedley Butler to lead it (he was a well-known anti-imperialist and social democrat; it would be like asking Elizabeth Warren to lead a coup on behalf of Wall Street), and nobody would have followed MacArthur after the Bonus March. ...
The stupidity of the embryonic Business Plot leaders is clearly illustrated by their approaching Butler. They were clueless about his politics and loyalties. Maybe the salesman who bought Butler lunch and a drink and flashed the cash thought Gimlet Eye could be bought like any other working class dupe. In any case they were so wrong its one of those stories lines that could never work in fiction, stupid to the point of silliness.