Campbell was returned on the recommendation of his first cousin, John, Duke of Argyll, whom he followed into opposition, voting against the Spanish convention in 1739. He was not put up again, for reasons apparent in a letter from one of his constituents about a by-election at Glasgow Burghs in 1744:
On Monday, there came one [letter] from the [3rd] Duke of Argyll, strongly recommending the heir of his family but we had one of the heirs formerly, and he wrote only three letters to this corporation [Glasgow] in seven years; and he did not so much as know who were our magistrates, for his first letter was directed to Richard Graham as provost of the town. So I assure you that recommendation will not take in this place at present.1Caldwell Pprs. ii(1), p. 62.
In later life he bought Liston Hall in Essex, near the Suffolk border, where he erected ‘an elegant modern building on the site of the ancient manor house’, since pulled down.2Wright, Essex, i. 562.
He died 8 Sept. 1787.