Since I went to the trouble to look it up:

There are estimated to be 4.4 million software developers employed in the U.S.[1] and about 150 million total jobs[2], so about 3% of employed persons in the U.S. are software developers.

About 11% of U.S. workers are union members[3], so there are more than 3 times as many union members as professional developers in the U.S.

[1] https://dqydj.com/number-of-developers-in-america-and-per-st...

[2] https://www.deptofnumbers.com/employment/us/

[3] https://www.bls.gov/news.release/union2.nr0.htm

Mostly public sector (around 1/3 unionized). Private is around 6%[1]. And after the public sector it's transport, utilities, telecoms, construction, manufacturing. Basically a laundry list of underperformance in the US -- important sectors (govt especially) choked to death by rent-seeking and complacency.

1: https://www.thebalancecareers.com/best-union-jobs-in-america...

For reference (for anyone curious) presently ~10% of US jobs are unionized. 40 years ago it was around to ~20%.

https://usafacts.org/articles/labor-union-membership/