He also joined the Social Democratic Workers' Party, becoming in 1923 one of its first members to win election to the Stadskanaal local council.
[1][2] In 1931, ter Borch began working for the union full-time, moving to Groningen, and was also appointed to its executive.
In 1940, he was appointed to the Groningen Provincial Council, as a replacement for F. J. Schaper, who was trying to maintain a low profile in the hope of avoiding persecution.
The councils were then reconstituted, ter Borch again taking his seat, and he also became prominent in reconstructing the trade union movement.
[1][2] Ter Borch became the chief administrator of the NVvFA in 1946, and then in 1950 became president of the renamed General Union of Miscellaneous Industries (ABC), moving to Amsterdam.