On 3 October 2019, Stewart announced he had resigned from the Conservative Party and that he would stand down as an MP at the next general election. He initially put himself up to be an independent candidate in the 2021 London mayoral election but withdrew on 6 May 2020 on the grounds of the election being postponed due to COVID-19, saying he could not maintain the campaign so long against the big budgets of the Labour and Conservative campaigns. In September 2020, he became a fellow at Yale University, teaching politics and international relations.