He was exempt from military draft for being indispensable as staff member of the Reformed Political League in the House of Representatives.

He specialised in foreign and military affairs, prepared the inquiry by the House of Representatives into the role of the Netherlands in the fall of Srebrenica.

In the 2006 elections, the Christian Union party doubled its seats and joined the fourth Balkenende cabinet.

In September 2008, he told the Dutch opinion magazine, Vrij Nederland, that he would have been very unhappy to have joined the army.

Because of this and other remarks, he received criticism from military labour unions, who said that Van Middelkoop had lost his credibility.