Until we fix our broken electoral system, I'm helping progressives beat the Tories by swapping their votes

For many people it's the only way to keep out David Cameron while protecting their party's share of the vote

Ed Miliband and Natalie Bennett are both 'progressive', but would you swap your vote for either of them?

Britain’s electoral system is an odd one. You can vote for the party closest to your own views. Or you can work out the most effective tactical vote against your least preferred outcome. And for voters in marginal seats this is a real dilemma.

You may be enthused by the pro-environment, anti-austerity politics of the Green Party, but are terrified by the idea of the cuts that would come with another Tory government – which a vote for Labour could help dilute. However, if you do vote tactically in this way, there is no way of letting your views be known. Your vote will be claimed by Labour as one of their own, rather than a "least-worst" tactical vote.

That is not an easy choice, but it may well be looked on with envy by the many more voters who live in constituencies that are safe for either of the big two parties. Here your vote has no chance of changing the government. All it can do is change the national share of the vote.

This is why a group of Labour and Green supporters have come up with the idea of www.voteswap.org. First, we want to make sure that progressive voters are equipped with all the best information about what kind of constituency they live in – is it safe, marginal or something more complex?

Then we want to give Labour voters in safe seats the chance to swap their votes with Green supporters in Labour marginal seats. This helps Labour win more MPs - helping remove the Tories from government, but without reducing the Green share of the national vote.

This means that Greens can tactically vote Labour without reducing their party’s overall tally, and Labour voters in safe seats can vote in a way that can help change the outcome of an election, perhaps for the first time in their lives.

So far it's been a huge success. Despite a very quiet launch last week with no media fanfare, the site has been visited just over 140,000 times in a few days, and nearly 5,000 people have pledged to swap their vote.

Of course not every seat is safe or a Labour marginal. The landscape is different in Scotland, Wales and Northern Ireland, so we only cover England.

In some seats only the Liberal Democrats and Conservatives can win, and we ignore these constituencies. But while the Lib Dems refuse to rule out a further coalition with the Conservatives, we are happy to recommend a Green or Labour vote in seats where they are challenging the Lib Dems.

Of course, the real solution to our electoroal problem would be to get rid of our first-past-the-post system. It makes less and less sense as we move away from a two and a half party system focussed on a "centre ground" that ignores issues important to millions. Part of the aim of the site has been to highlight just how broken the system is.

But this is still a general election where the result matters hugely. There are tribal Labour and Green activists who will condemn us. Yet our success so far suggests that there are enough shared values for many voters to understand this is win-win for progressives. By swapping their vote, the Conservatives lose more seats, but tactical voters do not have to harm their favoured party. And until our system is fixed, what's not to like about that?

Joe Cox is a political activist and a member of the Voteswap.org team