I came across some interesting notes on the song in the liner notes from Smithsonian/Folkways album Number FW05285 )click) titled The Original Talking Union and other Union Songs, by the Almanac Singers with Pete Seeger and chorus:

"Which Side Are You On?" is a miner's song written in 1932 in the midst of the bitter struggles of the miners in Harlan County, Kentucky. The mine owners, unyielding in their opposition to any
unionization of the workers, carried on a campaign of violence and terror to smash the union. At least a dozen miners were killed by deputies hired by the operators. but none of the deputies were indicted.
"Which Side Are You On" was written during one of the many terroristic raids by the sheriff and his deputies on the miners' homes. They came to the home of Sam Reece, one of the leaders of the
National Miners' Union, but he had been warned in time and escaped. They poked their shotguns everywhere, under the beds and into the closets, even into the piles of dirty linen, searching for the
miners' leader. When Reece's young daughters, aged 8 and 11, started crying, one of the deputies laughed and said: "What are you crying for? We don't want you . We're after your old man."
After the deputies had left, Mrs. Florence Reece, wife of the rank and file leader, was seething with indignation. She tore an old calendar off the wall, and on the back side wrote the verses of the great labor song, which she put to the tune of an old Baptist hymn she had known from childhood, The song was immediately picked up by the striking miners after it had been sung at the union hall by Mrs. Reece's two little girls. From Harlan County, it spread throughout the entire labor movement.

A few of the verses of the song were slightly changed later by the Almanac Singers:
"My daddy was a miner, and I'm a miner's son
And I'll stick with the union until the battle's won."
originally read:
My daddy was a miner; he's now in the air and sun.
He'll be with you. fellow workers, until this battle's won.
The words "he's now in the air and sun" refers to the fact that he was blacklisted from work.

Here are the lyrics, as performed by the Almanac Singers:

WHICH SIDE ARE YOU ON
(Florence Reece)

Come all you good worken,
Good news to you I'll tell
Of how the good old union
Has come in here to dwell.

Which side are you on?
Which side are you on?

My daddy was a miner*
And I'm a miner's son.
And I'll stick with the union
'Til every battle's won.

They say In Harlan County
There are no neutrals there;
You'll either be a union man
Or a thug for J.H. Blair.

Oh workers can you stand it?
Oh tell me how you can.
Will you be a lousy scab
Or will you be a man?

Don't scab lor the bosses,
Don't listen to their lies.
Us poor folks haven't got a chance
Unless we organize.

*originally read:
My daddy was a miner; he's now in the air and sun.
He'll be with you. fellow workers, until this battle's won.