Advisers for Donald Trump have floated the idea of demanding NATO allies spend 3% of gross domestic product on defense, a higher target than alliance members have agreed to and a warning shot that signals even more tension if he returns to the White House.

NATO’s primary tenet is that an attack on one member is an attack on all. Donald Trump has made clear that he won’t uphold that core principle, putting the future of NATO at risk if he’s re-elected.

The term "Trump-proofing" has become a common expression among pundits, academics and reporters amid the rising questions over the future of the support when a new U.S. president is sworn into office in January.

The 3% of GDP figure - just an idea for now, and not yet formal policy - could signal more tension with allies if Trump wins the US election Advisers for Donald Trump have floated the idea of demanding Nato allies spend 3 per cent of gross domestic product on defence,