With Britain's noisy campaign over a possible Brexit polarizing voters, tight referendums over EU membership two decades ago in Norway and Sweden offer lessons how campaigns that stay positive and focus on ordinary people can swing voters at the last minute.

In the space of just two weeks in 1994, Sweden voted by 52.3 percent to join the EU, while Norway voted by 52.2 percent to stay out. They were two of the closest-run EU referendums in history, momentous decisions that split families, generations and political parties.

Both nations are relevant to the British campaign. Norway is touted as a model by Brexit supporters, with close EU trade ties despite remaining outside. Sweden is seen as a successful arm's-length approach to EU membership, retaining its own currency.