Lewis served as Chair of the Defence Select Committee from 2015 to 2019. He actively pursues the retention and renewal of the British strategic nuclear deterrent, the UK Trident programme – confirmed in 2016 – and campaigns for Defence expenditure to be restored to 3% of GDP. Lewis had the Conservative Party whip removed after successfully standing against the Government's preferred candidate for the chairmanship of the Intelligence and Security Committee, former Secretary of State for Transport Chris Grayling, on 15 July 2020. The whip was restored on 30 December 2020.