How could an alliance between the Green Party, Liberal-Democrats and Labour Party be formed?

Question

The Green Party co-founders have written a letter to Jeremy Corbyn (Labour) and Tim Farron (Liberal Democrats) requesting to form an alliance:

We’d like to meet to explore the best options for beating the Tories in June. We understand that, in the immediate run up to an election, signalling a willingness to work with other parties might be difficult but we hope you’ll agree that the times we are living in require leaders to be courageous and visionary, to actively build a more positive politics

What political process would they need to forge the proposed alliance to attempt to 'defeat' the Conservatives on June 8th (if the snap election goes ahead)?

Answer 1

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.

Answer 2

There are two parts to this, the election and the actual parliament.

In the election they could in theory go as far as agreeing not to contest the same seats. Less extreme than that they could agree not to try and take seats from each other and focus all their campaign effort on constituencies where they were the strongest non-tory option.

AIUI After the election then if one party has an absoloute majority in parliament they get the role of government by default. Otherwise the sitting government remains in power until parliament decides otherwise.

We saw this happen back in 2010. No party won an absoloute majority so Labour remained in government in the immediate aftermath of the election. Then the tories and the lib-dems agreed to form a coalition. Once it became clear that the tory-lib dem coalition had the support of the majority of parliament and Labour did not Gordon Brown went to the queen to tender his resignation and ask her to invite the leader of the opposition to form a government.

Answer 3

How could a possible alliance between the Green Party, Liberal-Democrats and Labour Party be possible to 'defeat' the Conservatives?

By offering a platform that appeals to the most voters.

You have to recognize that intelligent voters vote on issues and policies that matter to them. Whether such policies are offered by their parties of affiliation matters little - putting aside those rabid fans.

So the only sure way for any part or party alliance is to put their own policies on the table and let voters to decide.

Labeling your opponents as "wrecking the country" is a sure way to lose, as Hillary found out recently.