He has been an MP since 1992, originally for the seat of Glasgow Govan and, following boundary changes, for Glasgow Pollok from 1997 to 2005. After further major boundary changes across Scotland, Davidson stood in the 2005 general election for the new constituency of Glasgow South West, where he won the second biggest swing to Labour in Scotland. He is a member of the Public Accounts Select Committee and has lobbied to support the Scottish shipbuilding industry. He is also secretary of the influential Trade Union Group of Labour MPs.
Like his colleague and friend Dennis Canavan, he was rejected as a candidate for the Scottish Parliament by the Labour Party, and has felt free to criticise the government since. In 2002 he criticised Prince Michael of Kent for 'squatting' in Kensington Palace. More recently he has criticised the Prince of Wales over his personal finances. He was the chair of Labour Against the Euro before it ceased campaigning following the 2003 decision by Gordon Brown that the five economic tests for the UK to join the euro had not been met.
During the debate in the House of Commons over the decision whether to have a referendum over the EU Treaty of Lisbon (5 March 2008), Davidson drew jeers from his Labour colleagues for branding New Labour supporters "Maoists and Trotskyists". Davidson was putting forward the case for disobeying the party line and voting for a referendum.
From 1973 to 1974, Davidson was Chair of the National Organisation of Labour Students.