Most of the criticism has been targeted at the plans to fund the expansion. The plan is a public-public partnership featuring at least one and possibly more countries for which Koper is a key port, notably Hungary.

The public appears to be fairly evenly split, with the latest polls suggesting turnout will be the decisive factor.

According to law, at least 20% of all voters must be against it in order to vote down a piece of legislation being challenged at a referendum.