Henderson served as the whip and was a leading member of the group of 11 SNP MPs in the House of Commons. In 1975, he negotiated a position for the SNP's Winnie Ewing in the British delegation to the European Parliament, where she became known as "Madame Ecosse". He was later the SNP spokesman on employment and industry. He was instrumental in the SNP's decision to lay down a censure motion in the House of Commons, criticising the Labour government of James Callaghan after 1 March 1979 referendum vote on Scottish devolution failed to be approved by the required 40% of the electorate. At the time, the Labour government were reliant upon support from the SNP. The motion was carried by one vote on 28 March and Callaghan immediately called the general election which brought the Conservative Party led by Margaret Thatcher to power. Callaghan described the actions of the SNP as "turkeys voting for Christmas". Henderson lost his seat to Conservative Albert McQuarrie in the 1979 general election by less than 600 votes, and only two MPs from the SNP remained in the new Parliament.