The constituency had been created from parts of Merton and Morden and Mitcham, both of which had usually generally returned Conservative Party MPs.

Although there was no requirement to do so, he decided to resign his seat and stand in the resulting by-election in order to gain a mandate for his change of allegiance.

[2] With a Conservative government in power, their share of the vote might have been expected to decline, but with the Falklands War in progress, their approval ratings were high, and the party hoped to make a decent showing.

The Labour Party, suffering from defections to the SDP, considered itself unlikely to hold the seat and stood David Nicholas.

The Liberal Party had agreed an electoral pact with the SDP the previous autumn and so did not stand a candidate against Douglas-Mann.

Douglas-Mann stood for the seat again in the SDP colours at the 1983 general election, but was pushed into third place by Nicholas.