Harrison was returned for Grimsby a third time on the interest of Charles Anderson Pelham in 1790. He had been admitted to Brooks’s Club by the same patron, 20 Apr. 1785. It does not appear that he was a member of the Whig Club. He continued to act selectively with the Whigs in opposition, but he had other roles. As previously, he supported repeal of the Test Act, in Scotland, in 1791. On 11 Mar., 4 and 11 Apr. and 27 May 1791 he was a spokesman for the agricultural interest in criticism of the corn bill: he was respected as an agricultural improver.
Harrison was listed a Portland Whig in December 1792 and ‘out of town’ when Fox divided the House against the address, 13 Dec.
Meanwhile his patron, having come to terms with ministers, was obliged to drop his Whig nominees and on 29 Dec. 1795 assured the Duke of Portland that Harrison, whom the Treasury was listing ‘con’, had ‘no intention of offering himself again for the borough of Grimsby’.
In the Parliaments of 1796 and 1802 Harrison was returned for Thetford on the Duke of Grafton’s interest: their politics were congenial. He at once resumed opposition, attacking the imperial loan, 8 Dec. 1796 (promising a motion to effect savings) and the acquisition of the Cape, 20 Dec. He gave notice of his motion on 24 Feb. 1797 and it was at length heard on 13 Mar. His aim was public retrenchment, with the abolition of sinecures. Pitt moved the previous question, promising that the finance committee he intended to set up would consider Harrison’s proposals. Harrison was defeated by 169 votes to 77. French Laurence assured Edmund Burke that this motion was designed only to achieve ‘little savings’, and not to reduce influence, like Burke’s ‘economical reform’. (A year later, Pitt suggested jokingly to Windham that the vacant Privy Seal had better be filled quickly, ‘lest Mr Harrison should discover that we could go on as well without any such office’.) Harrison had to console himself with a place on the finance committee. He attended regularly and helped particularly with its reports on the transport office and on the barrack department.
Harrison was unimpressed with the replacement of Pitt by Addington and tried to obstruct the grant of supply until the new ministry was visibly established, 16 Feb. 1801. Thereafter he had little to say, but continued to vote with opposition. He did complain of the prodigality of the Irish master of the rolls bill, 19 Mar., of the burden of the poor rates, 21 Mar. 1801, and of the precipitate, unprofessional nature of the debtors bill, 21 May 1802. He joined in the vote of thanks for the removal of Pitt from office, 7 May 1802. He was an advocate of the application of the duchy of Cornwall revenue to the Prince of Wales’s debts, 23 Feb. 1803, and voted in this sense, 4 Mar. After leaves of absence, he voted with opposition on the resumption of hostilities, 24 May 1803, and silently opposed Addington’s defence measures in April 1804. Listed a Foxite, he opposed Pitt’s additional force bill (though not apparently until 18 June 1804) and voted against the war with Spain, 12 Feb. 1805. He was in the majorities for censure and criminal prosecution of Melville, 8 Apr., 12 June 1805, and opposed the Duke of Atholl’s claims to compensation, 19 June. Next day he accused the law officers of prevarication in calling on the House to define the case against Melville, which he regarded as their professional duty. He played little part in the session of 1806. On 4 Feb. he pointed out a defect in the malt duty bill and he was a spokesman for British wool producers against the bill to permit the import of wool from British America, 4-7 Mar.; after that, nothing. He retired from ‘age and infirmity’ at the dissolution and it appeared that he could not be tempted to return to Westminster.
