However, he gradually developed a considerable hatred against the king, and after his resignation as chamberlain he joined the opposition in the Riksdag of the Estates.
He was the first to proclaim in the Rikssalen at Stockholm Palace that the Act of Union and Security proposed by King Gustav III needed to be discussed separately by the four Estates.
The act essentially provided King Gustav III tremendous power and simultaneously dramatically reduced the rights and privileges of the nobility.
Hamilton was initially much in favor of the French Revolution, however, after observing the tyranny during the Reign of Terror his feelings towards Jacobinism became lukewarm and in 1800 he had reversed his position and became a royalist.
In his later years, probably in the year 1795, he completed the book Anecdoter till Gustaf III:s och Gustaf IV Adolfs historia, which contains many anecdotes about the influential people in the mid-18th century Sweden.