In 1780 Middleton was defeated at Ipswich after a hotly contested and expensive election. In 1784 he stood again, supported by Administration, and topped the poll. He voted for parliamentary reform, 18 Apr. 1785, and supported Pitt on Richmond’s fortifications plan, 27 Feb. 1786. His only reported speech in this Parliament was on the resolutions to grant money for the relief of American loyalists, 18 May 1786, when he declared he would ‘endeavour to prevent imposters from receiving any part’, and ‘in order to discriminate between the deserving and the undeserving’ moved for a list of those receiving pensions or allowances.
Middleton died 26 Dec. 1829. Oldfield describes him as ‘a representative whose independence was an ornament to the British Senate’.