Roper was elected Member of Parliament for Farnworth in 1970. He sat as a Labour Co-operative MP from 1970 to 1981, and for the Social Democratic Party (SDP) from 1981 to 1983, when he was also the party's Chief Whip. His Farnworth seat was then abolished, and he contested the seat of Worsley at the 1983 general election. He finished third in a three-way marginal election.