SDP-Liberal Alliance
Electoral alliance of the Social Democratic Party and the Liberal Party, 1981 to 1988.
Fragmented 2026 polling, with Reform UK on 25% in YouGov's 5-6 July survey and four parties dividing the rest, has made the Alliance the standard warning: it led national polls in 1981, then turned 25.4% of the 1983 vote into 23 seats.
Last refreshed: 7 October 2026
Timeline for SDP-Liberal Alliance
Mentioned in: Labour takes Hirst and Amble from Reform
UK Local Elections 2026Mentioned in: Reform holds 25% through the storm
UK Local Elections 2026Mentioned in: Reform projected to 2,342 council seats
UK Local Elections 2026Mentioned in: Labour and Greens draw level at 16%
UK Local Elections 2026Background
Founded in 1981, the Alliance joined the Social Democratic Party, which Labour defectors including Roy Jenkins had launched that year, to the Liberal Party. It stood at the general elections of 1983 and 1987, and in 1988 its two members merged into the party that became the Liberal Democrats.
In 1981 the Alliance also broke through locally, winning council seats across southern England, but that support drained back once the 1983 contest narrowed to a two-way choice. At that election it and Labour shared the anti-Conservative vote almost equally, and Margaret Thatcher won a landslide on a lower Conservative share.
the Alliance remains the reference case for one side's vote dividing under First past the post. In October 2026 the comparison moved to the right, after Reform UK and Restore Britain together took 40.2% in Amble, Northumberland, against the winning Labour candidate's 38.8%.
The Alliance peaked too early
Analysts reached for the Alliance as British polling splintered in 2026. When YouGov put Labour and The Greens level on 16% on 7 April, the comparison was its run of 1981 and 1982, when it polled above Labour, touched 50% in the Gallup aggregate and won the Crosby by-election in November 1981. None of that survived the general election: in 1983 First past the post gave it 23 seats for 25.4% of the vote.
Its record was read across to Reform UK, steady on 25% in YouGov's 5-6 July poll. The lesson drawn was that an insurgent's early poll strength can drain away long before voters face a real ballot, as the Alliance's December 1981 peak did.