Britain’s first-past-the-post electoral system means that lots of MPs are elected on fewer than half of the votes cast in their constituencies.
At the last election, 333 out of 650 winning candidates received less than 50% of the vote.
Those MPs could have been defeated if all the people who voted against them had gone for the same alternative candidate.
Clearly, that would never happen everywhere. But parties who agree on some of the main issues potentially have a lot to gain if they can co-ordinate their supporters.
One option is to encourage tactical voting. A more direct approach is to enter some sort of alliance or pact which sees parties choosing not to put forward candidates in certain seats.