British Prime Minister David Cameron couldn't have asked for a better start to the campaign to keep Britain inside the European Union.
On Sunday, U.S. President Barack Obama completed a three-day visit to London during which he spent much of his time bolstering the case for Britain to remain in the EU and raising fears about what would happen if the country pulled out.
Just before leaving, Mr. Obama told the BBC that if Britain left the EU it could take the country up to 10 years to negotiate a trade deal with the United States. On Friday, he told reporters Britain would be "at the back of the queue" when it came to a U.S. trade agreement.
Mr. Obama's interventions capped off the first week of official campaigning for the June 23 referendum on whether Britain should stay in the EU and they left Mr. Cameron beaming. Mr. Cameron is a leading figure on the Remain side and the President's comments cut to the heart of the arguments of those advocating for a so-called Brexit.
They have argued Britain would be better off outside the EU in part because the country could negotiate its own trade agreements around the world. Mr. Obama poured cold water on that suggestion, saying the United States is concentrating on negotiating a deal with the EU.
"The U.K. would not be able to negotiate something with the United States faster than the EU," he told the BBC. "We wouldn't abandon our efforts to negotiate a trade deal with our largest trading partner, the European market."
Campaigners for the Leave side were left trying to discredit Mr. Obama, saying he wouldn't be president much longer and a new administration would take a different view. That claim became difficult after a spokesman for Hillary Clinton, who is leading the race to become the Democratic Party presidential nominee, said on Sunday that she too supports Britain remaining in the EU. And Republican front-runner Donald Trump has been suspicious of all trade deals.
The Leave side also wasn't helped by the comments of Conservative MP Boris Johnson, a prominent Brexit campaigner who targeted Mr. Obama's ethnicity and wrote in a British newspaper that the President's "half-Kenyan" ancestry made him so anti-British he removed a bust of Winston Churchill from the Oval Office. The comments drew harsh criticism in Britain and Mr. Obama shot back that he loved Mr. Churchill but felt that as the first African-American president a statue of Martin Luther King Jr. was more appropriate.
On Sunday, Mr. Johnson tried again and slammed Mr. Obama's views on trade.
"It is ridiculous to warn that the U.K. will be at the back of the queue for a free-trade deal," Mr. Johnson told the Daily Mail. "The U.K. has never been able to do a free-trade deal with the U.S. in the last 43 years – because we are in the EU."
Mr. Johnson and others on the Leave side have also argued that the United States would never give up as much sovereignty as Britain has had to as a member of the EU.
So far those arguments appear to be losing ground and polls show the Remain side winning over voters as concerns increase about the country's economic future outside the EU. Last week, Bank of England Governor Mark Carney, the British Treasury department and eight former U.S. secretaries of Treasury all said Britain would face serious economic challenges if it voted to leave the EU.
A poll released last week by ComRes for the Daily Mail and ITV News showed 51 per cent of 1,002 people surveyed would vote to stay in the EU. That was up three percentage points from a similar poll in March. The percentage of those saying they would vote to leave fell one point to 40 per cent. The remainder didn't know.