Tom Donilon - who spent three years as the most senior voice on foreign policy in the Obama administration - was clear. On issues where America dealt directly with the EU, issues like the nuclear deal with Iran, immigration and aid spending, not being present at the EU table would lessen Britain's importance for the US.
But Mr Donilon's overriding concern was not one of specific foreign policy issues it was a broader question of focus. Voting leave, he fears, would lead to a Britain that is more inward looking and more distracted from global affairs.
That, he added, would be a less powerful partner for the US. On a string of critical issues, Britain would be less important to the White House if it were not part of the European Union.
I sat down with Mr Donilon, President Barack Obama's previous National Security Adviser, after he and a group of senior American foreign policy officials signed an open letter urging Brits to vote to stay in the EU on June 23rd. He acknowledged that it was unusual for American officials, from the president down, to weigh in on a British political issue. However Mr Donilon felt the stakes are so high for Britain, the EU and the US that it was important to do so.
How would the relationship between the White House and the UK change if Britain were no longer part of the EU?