Single-issue group and Special interest

Special interest and single-issue group are semantically related In some cases you can replace phrase "Special interest" with "Single-issue group", this phrases are similar.

Special interest noun - A group of people with a common identifying interest that they seek to protect and promote.

Single-issue group and special interest are semantically related in special interest group topic. You can use "Single-issue group" instead a noun phrase "Special interest".