Parliamentary career
Lewis stood as the Conservative candidate in Swansea West at the 1983 general election, coming second with 36.6% of the vote behind the incumbent Labour MP Alan Williams.
At the 1997 general election, Lewis was elected to Parliament as MP for New Forest East with 42.9% of the vote and a majority of 5,215.
In August 2000, Lewis attacked Conservative defectors to Labour due to the party supporting Section 28, describing the "gay lobby" as supporting "propaganda promoting homosexuality" at the expense of taxpayers.
Lewis was re-elected as MP for New Forest East at the 2001 general election with a decreased vote share of 42.4% and a decreased majority of 3,829. He was again re-elected at the 2005 general election, with an increased vote share of 48.6% and an increased majority of 6,551.
Although Lewis was repeatedly listed as amongst the lowest-claiming MPs (ranked 566th out of 647 in 2008/09), The Sunday Telegraph alleged in May 2009 that he had tried to claim the £6,000 cost of a wooden floor in his second home. He maintained that: "At no stage did I claim for the flooring and it did not cost the taxpayer a penny." A senior Commons official confirmed that, by seeking advice in advance about second home expenditure, he had acted "in accordance with best practice as recommended by this department" and that "it is not true that you attempted to claim £6,000 in expenses for a wooden floor at your second home".
At the 2010 general election, Lewis was again re-elected with an increased vote share of 52.8% and an increased majority of 11,307.
With the creation of the Liberal-Conservative Coalition as a result of the election of a hung parliament in 2010, the post which he had shadowed (Minister for the Armed Forces) was allocated to the Liberal Democrat Defence spokesman, Nick Harvey MP. Lewis was appointed as a member of Parliament's Intelligence and Security Committee in September 2010. He has also been a vice-chairman of the Conservative Friends of Poland.
Lewis has been described by The Daily Telegraph as "one of the most vigorous rightwingers in the Commons" and by The Guardian as the Conservative Party's "front bench terrier". He was one of the Frontbenchers and Backbenchers of the Year chosen by commentators on the ConservativeHome website, in December 2009 and December 2010 respectively.
In February 2011, he strongly opposed, and was one of three Conservative MPs who voted against, Coalition plans to transfer heritage forests from public ownership to trusts. The plans were later disowned by the Government and abandoned.
In October 2011, he was one of 81 Conservative rebels who voted in favour of a referendum on the UK's membership of the European Union and, in October 2012, he was one of 53 Conservative rebels voting to demand a real-terms cut in the EU budget. Both policies were later adopted by the party leadership.
In May 2014, he was one of eight candidates for the chairmanship of the House of Commons Defence Select Committee, coming second with 212 votes to the eventual winner's 226.
At the 2015 general election, Lewis was again re-elected with an increased vote share of 56.3% and an increased majority of 19,162.
On 17 June 2015, he was elected to chair the Defence Select Committee in a three-way contest with Richard Benyon and Colonel Bob Stewart.
In November and December 2015, before and during the debate on bombing ISIL/Daesh in Syria, Lewis challenged David Cameron's claim that there were 70,000 "moderate" Syrian fighters opposing ISIL/Daesh, describing the figure as "magical" and quoting expert commentators' views that the "Free Syrian Army" contained many Islamists. In the Commons debate on 2 December, Lewis stated that "instead of having dodgy dossiers [as in the 2003 Iraq conflict], we now have bogus battalions of moderate fighters". He predicted that "Once Daesh has been driven out... an Occupying Power will have to remain in control for many years to come... and only the Syrian Government Army is likely to provide it... Airstrikes alone are a dangerous diversion and distraction. What is needed is a grand military alliance involving not only the West but Russia and, yes, its Syrian Government clients too.... We need to choose the lesser of two evils and abandon the fiction of a cosy third choice" between "very nasty authoritarians and Islamist totalitarians". After making this widely reported speech, Lewis voted against extending airstrikes against ISIL/Daesh into Syria in the absence of "credible ground forces", and he continued to maintain that, apart from the Kurdish-led forces, in Syria the choice remained "between monsters on the one hand, and maniacs on the other".
Lewis was again re-elected at the snap 2017 general election with an increased vote share of 62.6% and an increased majority of 21,995.
On 12 July 2017, he was again elected to chair the Defence Select Committee, by 305 votes to 265 votes, in a direct contest with Johnny Mercer.
He was again re-elected at the 2019 general election with an increased vote share of 64.5% and an increased majority of 25,251. Lewis was again re-elected at the 2024 general election, with a decreased vote share of 38.5% and a decreased majority of 8,495.