Cornewall considered himself safe in the county seat he owed chiefly to his wife’s family. He had been in opposition to Pitt and was surprised that the Whig recovery was not greater in the election of 1790. To his brother-in-law Lord Malmesbury he wrote, ‘I hope your opinion as to peace is well founded; in mine, nothing is ever to be got by war, even the most successful’. He voted against Pitt’s bellicose Russian policy, 12 Apr. 1791, but in the same month was listed absent and hostile on the forthcoming motion to repeal the Test Act. In December 1792 he was a Portland Whig; on 17 Feb. 1793 according to Lord Sheffield, he was present at Windham’s ‘third party’ meeting and, next day, he ‘strongly urged the necessity of resisting the progress of French aims and principles’ in the House. In company with his brothers-in-law (the other was Sir Gilbert Elliot), he went over to ministers, though he spoke and voted for delay of the transportation of the radical cleric Thomas Fyshe Palmer, 24 Feb. 1794. On 30 Oct. 1795 he informed Elliot, ‘I, now, ought to be in London ... but, having had my full dose last year, I mean to play truant this ... unless a call of the House forces me up’. He wrote to Pitt for local patronage twice that session.
Cornewall was shocked when he was ousted by a revanchiste Whig at the poll in 1796, styling himself one of ‘the Duke of Portland’s martyrs’. Having spent nearly £2,000, he dreaded a ‘constant canvass’ to recapture his seat for himself or his son George and, still more, ‘a real thorough contest’. It looked as if he would slip into the seat quietly on the retirement (disclosed privately to him) of Thomas Harley, county opinion having swung in his favour; but he faced another unexpected contest in 1802. He defeated the Whig Member, but was embarrassed by a tactical petition against him for treating, in the wake of another which unseated the other sitting Member on that ground. Elliot came to the rescue in securing its rejection: Cornewall’s election was confirmed.
In August 1805 Cornewall was reduced to a nervous wreck when it was discovered that the family bank in which he had been a partner for nearly 30 years was ‘on the brink of insolvency’. He had withdrawn most of his capital to redeem a war-ravaged estate in Grenada, but his brother Thomas Amyand had just died owing the bank £20,000. If the bank were to be liquidated Cornewall stood to lose £20,000; to save it he must borrow £15,000 at once. This was not easy, as most of his estates were settled. (They brought him £4,500 p.a. and Grenada £2,000 more.) Demoralized, he relied on Lord Minto to raise the funds to save the situation.