On his second attempt, Thomas gained the seat at that election, which took place in October 1974. He won Bristol North West by just 633 votes, and a majority of 1.2%, approximately the same share he had previously lost by. There was a swing to Labour of 2.3% (higher than the national swing of 2.0%), and Labour won a small majority in the House of Commons, where Thomas became the chairman of the Tribune Group of left-wing Labour MPs. In 1975, he was part of a joint statement made by Bristol Labour MPs advocating a vote against membership of the European Communities in that year's UK referendum.