I've decided to set my campaign in Northern Illinois and Indiana, southern Michigan and Wisconsin. You'd have Chicago, and the Progressives and Socialists of Wisconsin, hemmed in by the Army in the west at Rock Island Armory (which could make tanks and artillery) and Henry Ford on the east. Both sides would have plenty of people and industry.
I could do land battles of all sizes (for our European friends the territory this area covers is about the size of France), air battles (Navy fighters vs. Ford Tri-Motor Bombers escorted by Army Fighters), even small sea battles in Lake Michigan (motorboats and armed merchant ships). Land, sea, and air the region has it all. Don't forget the winters that can be as bad as Russia. OK maybe not that bad but plenty of opportunity for ski troops and aerosans (I've been looking for an excuse to buy some of those from Warlord anyway).
The Black Legion was a real organization that had splintered from the Ku Klux Klan. The organization was Founded by William Shepard in east central Ohio. The group's total membership, estimated between 20,000 and 30,000, was centered in Detroit, Michigan, though the Legion was also highly active in Ohio and one of its self-described leaders, Virgil "Bert" Effinger, lived and worked in Lima, Ohio. After a high profile murder in 1936 that saw some leaders go to jail (they made the mistake of killing a white man who wasn't a union organizer or even a leftist) the Legion quickly declined. Humphrey Bogart starred in the movie the Black Legion loosely based on that murder in 1937.
The Associated Press described the organization on May 31, 1936, as
"a group of loosely federated night-riding bands operating in several States without central discipline or common purpose beyond the enforcement by lash and pistol of individual leaders' notions of Americanism."
The Black Legion was organized along paramilitary lines and had five brigades, 16 regiments, 64 battalions, and 256 companies. Although its members boasted that there were one million Legionnaires in Michigan, it probably had only between 20,000 and 30,000 members in the state in the 1930s, one third of whom lived in Detroit.
Members wore black Klan-style outfits with skull-and-crossbones insignia, and were allegedly responsible for numerous murders of alleged communists and socialists.
So the regular Black Legion would be a fast truck-born (thanks to Henry) mobile strike force, well equipped with small arms, LMGs, BARs and SMGs. No artillery, but some air support (Ford Tri-Motors converted into light bombers). Usually equipped with a couple armored vehicles, there would be a 10% chance (each) that they won't be available for a battle (worker sabotage). Note, the air support and armor belongs to Ford and is manned by "his" mercenaries. He just lends it to the Black Legion (more on Ford's Army in a future post).
They'd have access to plenty of weapons, but would not well trained (maybe a few combat veterans of WWI), with leadership determined by personal charisma and political abilities rather than military skill. That will be true of many of the non-military armed groups. The leader is good at making speeches and is dedicated to certain political beliefs, not necessarily good at tactics or other military skills. As the war goes on these political "generals" will be replaced by the competent, if their factions want to survive (early in the war you will have to roll for some of your leaders, they might be good, possibly great, possibly utter cowards who run at the first shot).