The Labour Party front bench has said that Britain remaining in the EU Customs Union would guarantee against a hard border in Ireland.
It is not true. To avoid a hard border requires Britain also to be in the Single Market, i.e. to have common economic regulations and free movement. Turkey has a customs union with the EU.
There is no free or even easy movement across the Turkish-EU border. It is a "hard border".
The Financial Times reported (16 February 2017): "A line of trucks 4km long stretched along the highway, inching along glacially towards the Bulgarian checkpoints. 'Today is a good day', said Ibrahim Kurtukcu, a 42-year trucker who had been waiting 14 hours. 'Last week the line was 7km long'. The record is 17km. It can take up to 30 hours to get through to the other side".
The Tories' worry is that the "backstop" obliges them to keep Northern Ireland in the Single Market indefinitely, so they can't change Single Market regulations in Britain without setting up a "hard" border in the Irish Sea.
Labour leaders have denounced the "backstop" as damaging. Actually, no. It's the best thing in May's formula. The democratic, pro-united-Ireland policy is to have the "backstop" apply to the whole UK.