A "Yes" vote for independence would be an economic mistake for Scotland and a geopolitical disaster for the west, senior U.S. figures – including Alan Greenspan – tell the Financial Times as Washington wakes up to the chance that its closest ally could break up this week.
Having assumed for months that "No" would win comfortably,Washington has reacted with alarm to opinion polls showing that Thursday's referendum is going down to the wire. "We have an interest in seeing the U.K. remain strong, robust and united," said Josh Earnest, the White House spokesman.
Mr Greenspan, former chairman of the U.S. Federal Reserve, said the economic consequences of independence would be "surprisingly negative for Scotland, more so than the Nationalist party is in any way communicating".
"Their [nationalist] forecasts are so implausible they really should be dismissed out of hand," said the normally circumspect Mr Greenspan, noting the pace of decline in North Sea oil production.
Despite Nationalist claims to the contrary, he said there was no chance of London agreeing to a currency union. Differing fiscal policies would also cause any Scottish attempt at using the pound regardless to "break apart very quickly".
"There's no conceivable, credible way the Bank of England is going to sit there as a lender of last resort to a new Scotland," said Mr Greenspan.
Many U.S. officials combine ancestral roots in Scotland and knowledge of the Scottish Enlightenment's influence on the U.S. constitution with strong emotional ties to the UK, an ally the U.S. has fought alongside for 100 years.
"Like many Americans, and given that my name is Robert Bruce, I have an admiration for the Scots, their heritage, and their role in U.S. and world history," said Robert Zoellick, the former deputy secretary of state and World Bank president.
"But a break-up of the U.K. would be a diminution of Britain and a tragedy for the west just at a moment when the U.S. needs strong partners. I strongly suspect it would not work out well for the Scots either."
Senator John McCain, a former Republican presidential candidate who regards the U.K. as Washington's most important military and intelligence partner, said he was reluctant to comment on an internal issue in another country.
"But I don't see how it could be helpful, not just as far as intelligence ties are concerned, but to the unique military relationship as well," he said.
The U.S. is especially worried that a Scottish Yes vote could increase the chances that the rest of the U.K. might vote to leave the EU, which U.S. officials believe would make Britain a much less potent partner.
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"That is our nightmare – Scottish independence followed by a British exit from the EU," said a senior administration official.
Another big issue for the U.S. would be the uncertainty surrounding the future of Britain's nuclear deterrent in the event of Scotland becoming independent.
The U.S. would like Britain to remain a nuclear power, but some officials worry about the ability of a smaller U.K. to sustain other conventional forces, while also paying for the nuclear weapons.
Foreign policy thinkers also fear that a Scottish Yes vote would embolden Russia – especially in Ukraine – even as it weakened the U.K. and thus Europe's ability to mount a united response.
"Russia could argue that separatist movements are actually perfectly legitimate, whether in Crimea or eastern Ukraine," said Ivo Daalder, former U.S. ambassador to Nato and now president of the Chicago Council on Global Affairs. "Of course, a democratic process that takes two years is not the same as an annexation and a fake referendum."