With Brexit on the horizon and the main parties so divided, it’s no wonder that progressive voters want to come together to show a united front. Tactical2017, a progressive campaign that encouraged people to vote tactically in the 2017 general election to get the Conservatives out of government, is being asked again for guidance, this time for the European parliament elections on 23 May.

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Here is why we won’t be making any solid recommendations, and why we advise people not to follow other sites that do.

Seat calculation

In England, Scotland and Wales, a form of proportional representation called the D’Hondt method is used for the European parliament elections. This means that the tactics used in a first-past-the-post (FPTP) system, such as the general election in 2017, do not work.

The UK is split into large regions and seats are assigned to each party as a proportion of the total vote. This system is more representative than FPTP but still results in many wasted votes as it favours large parties. So although we can’t apply the usual tactics to our vote, we should still think about whether or not a vote for our preferred party might be wasted.

As a rule, any party at least as popular as the Greens is worth voting for if you want to make sure your vote goes for a party that has a chance of securing a seat. This includes Plaid Cymru in Wales and the SNP in Scotland. Does it include Change UK in any region? It’s very difficult to say.

Unfortunately, this means that votes for small parties such as the Yorkshire party or independents would likely be a “wasted” vote because they do not have a significant chance of winning a seat. This isn’t to say that you shouldn’t vote for them, but if you want your vote to have the most chance of electing a progressive candidate, electing a remain candidate, or preventing a rightwing candidate, then your vote would be better going to a more popular party.

A changing political landscape

Another huge barrier against tactical voting in the European parliament election is the drastically changing political landscape in the UK. Usually we would look to the previous election for guidance, but UK politics has changed so drastically since then that the numbers are almost meaningless. Both Change UK and the Brexit party weren’t in play in 2014, Labour’s supporters are now divided on their Brexit stance and the Conservatives have seen huge losses in popularity if the local elections are any indication. This plus the D’Hondt method unfortunately makes it impossible to predict the figures with enough accuracy to make useful tactical voting recommendations.

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There are opinion polls that give a rough idea of how the country might vote and there are some sites out there trying to make sense of it all and use this information to advise people which way to vote. But how we vote in this election does seem to depend on how one feels about Brexit. In the general election it just so happened that voting in the most progressive candidates in each seat was also the best strategic way to stop Brexit. But this time, it’s different.

The Labour conundrum

For a lot of voters, the most important issue in this election is Brexit. In order to show the government that there is a strong will to remain in the EU, many people will be voting for a party that is explicitly pro-remain. For some that means they won’t be voting Labour. This begins to split the progressive vote and causes even more difficulty for tactical voting sites. If the aim of the site is to simply give the most remain-friendly election result, what should they advise in a region if Labour is the party in the best position to prevent a Brexit party seat?

How many people will be using their vote to show their stance on Brexit is hard to say, and it isn’t just remainers. Nigel Farage’s Brexit party is performing well in opinion polls but no one knows how its figures will compare to Ukip’s in 2014 and whether it might see increased support from Conservative or Labour voters who don’t think either party is pro-Brexit enough. The irony that they might lose supporters for being both too pro-Brexit and too anti-Brexit aside, this is yet another layer that tactical sites would have to take into consideration.

The consequences of getting it wrong

When tactical voting is attempted in a system using the D’Hondt method it is extremely important to be accurate. If the numbers are right then it might cause a remain party to take a seat from a leave party. But it’s also possible to result in fewer remain seats if the numbers are off, and as I have already established it is next to impossible to accurately predict the numbers for this election. This is why Tactical2017 won’t be telling you who to vote for.

Tactical voting recommendations can also suppress voter turnout if they are advised without confidence. The numbers are confusing and the conclusions are unclear. Advising somebody to vote a certain way when they don’t understand why can put them off. Especially when different sites give different advice, or other progressive parties tell them that it is in fact their party who should get the tactical vote.

Some of these problems can be reduced by recommending multiple parties in each region instead of just one, but it doesn’t solve every problem – and I haven’t seen any sites doing this so far.

Every vote helps

The bottom line is, the more progressive votes we get, the more progressive seats we will win. Getting more progressive voters to the polls should be the only “tactic” we use in 2019. This is the huge advantage of using the D’Hondt method. It means that Labour, the Liberal Democrats and the Greens can all win seats in the same region rather than the “winner takes all” approach of the FPTP system used in the general election. Voter turnout in 2014 was only 35.6% which is around half the usual turnout for the general election. This gives us a huge opportunity to completely change the result simply by persuading more people to vote.

In South West England, for example, it is possible for both the Lib Dems and the Green party to win a seat. If the local elections are anything to go on, the Conservatives may have lost enough support to lose a seat in that area, and if voter turnout is a little bit higher than last time then both the Lib Dems and the Greens can win a seat.

Instead of trying to advise people whether they should vote Lib Dem or Green, we should be advising them to bring their friends and family to the polling stations. Instead of trying to convert Green voters to Lib Dem, or Labour voters to SNP, we should be looking to convert non-voters to voters.

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Real tactics

I don’t want to put people off thinking about their vote carefully. It is likely that the overall votes for remain and leave parties will be compared, and you may wish to choose a party whose stance on this is clear. And I don’t want to mislead anyone into thinking that a constituency such as North East England, with only three seats available, would be somewhere all progressive parties could win a seat. I’ve spent a lot of time trying to persuade people to vote a certain way, and I will certainly be doing so again in the next general election. But for now I just want to advise as many people as possible to vote on 23 May – whoever it’s for.

• Becky Snowden is the founder and spokeswoman for the tactical voting site Tactical2017.com