They could agree to not stand in the same constituencies, like for the partial PS-EELV (red-green) agreement in France.
For example, they could decide that in each constituency, only the candidate for the party that received the most votes (between the three of them) would stand. The two major unionist parties in Northern Ireland, DUP and UUP had such a part in some constituencies in the 2015 elections, and look likely to have a similar pact in 2017.
This is particularly relevant in a first-path-the-post winner-takes-all system. In many constituencies, the largest party received less than 50% of the vote:
- Hypothethical example: If the non-Conservative vote is split between Labour, Lib.-Dem., and Greens with 24% each, a Conservative candidate could win with only 28% of the vote despite 72% preferring a non-Conservative candidate.
- In 2015, in Belfast South, SDLP won with only 24.5% of the vote. DUP had 22.2%, UUP had 9.1%. Had the DUP-UUP pact worked in this constituency, one of them would probably have received more votes than SDLP and won the seat.
- Also in 2015, in Portsmouth South, Conservatives won with 34.8%, Lib. Dem. 22.3%, Labour 19.5%, and Green 7.5%. Conservatives gained the seat from Lib. Dem. although their share of the vote increased only from 33.3% to 34.8%. An electoral pact such as proposed by the Green Party could possibly have prevented a Conservative gain there in 2015.
In the French legislative election, 2017, the Parti Socialiste withdrew their candidates from 42 constituencies where the Greens are in charge or have a good chance of winning, and the Greens reciprocally withdrew in 53 constituencies. A similar agreement exists between centrist parties En Marche and Mouvement démocrate.
An electoral system that could reduce the need for such a pact would be a ranked voting methods, such that Unionists could rank their preferred party 1 and the other Unionist party 2, or that a progressive voter could, for example, rank Green / Labour / Lib. Dem. Such a system was rejected in a 2011 referendum.