The rival campaigns officially kicked off this week in Britain ahead of a referendum scheduled for June 23 on whether the country should stay in the European Union.
Polls show the public is split 50-50 on staying in or leaving the bloc.
The "In" campaign — full title "Britain Stronger in Europe" — was seemingly given an early boost by a warning from the International Monetary Fund of "severe regional and global damage" if Britain voted to leave. The threat is real, argues Simon Tilford of the Center for European Reform, a broadly pro-European analyst group.
“We could see the EU fracturing and becoming a much less influential actor globally,” he said. “If it does so, it will weaken one of the West’s two big centers of power, the EU and the United States. So it will make it harder to address global problems, it will leave the U.S. more isolated than it currently is, and it will embolden other centers of power in the world, be it Russia, China.”
The head of the World Bank, Jim Yong Kim, added his voice to concerns over a British exit from the EU.
“This is an economy, given our projections of lower growth this year, that is not going to do well with more uncertainty,” he told reporters Thursday.
The "Out" campaign rejects that argument. One of its lead figures is London Mayor Boris Johnson, who told lawmakers earlier this month that Britain “should get out from under that system and have a free-trade arrangement that continued to give access to U.K. goods and services on the European continent.”
Individual trade pacts
The EU is the largest single market in the world. Pro-EU business groups say Britain would have to renegotiate individual trade deals in the event of a British exit.
“To try to renegotiate those locally for the U.K., given the size of trading relationships with the U.S. and with the Far and Middle East, I think those would be incredibly challenging,” said Elizabeth Shanahan of the Irish International Business Network.
She added that London should not take for granted its position as Europe’s financial capital.
“If there is a Brexit, which hopefully there won’t be, but I would the think the day after, I can imagine that the finance ministers of France and Germany will get together and they’ll work out exactly where the financial hub will be," Shanahan said. "And it won’t be here in London.”
So-called "pro- Brexit" campaigners say Britain could build stronger trade links with countries outside Europe, like China and the United States.
But U.S. President Barack Obama is expected to reiterate his support for Britain staying in the European Union during a visit to London next week.