If the political opinions are reduced to a spectum running from left to right, then in a two party system the major parties will sit immediately to either side of the mid-way point, because they want to seduce as many swing voters as possible from the other side.
As in, the conservative party may have conservative dreams but their policies need to be far enough to the left to actually win an election, so they will push up against the progressive party.
If suddenly more voters vote progressive (the conservative party loses badly), the conservative party needs to adjust their policy settings further to the left.
As the conservative party’s policies move to the left, the progressive party will start losing voters to them unless they also move further to the left.