Stewart apparently became a supporter of John Campbell, 2nd Duke of Argyll, and was elected to represent Malmesbury on his interest, on 25 January 1723. He represented Malmesbury until 1727, and though Sir Charles Wager expected that Argyll would arrange for Stewart to be elected to represent Portsmouth in 1732, this did not happen. Stewart's next opportunity to distinguish himself at sea came in late 1729, towards the end of the Anglo-Spanish War. News arrived in England that Edward St. Lo, commander in chief at the Jamaica Station during the Blockade of Porto Bello, had died while in command. Stewart was promoted to rear-admiral of the blue on 9 December 1729 and set out to take up the command in Lo's stead, flying his flag aboard .