A man wears a multitude of “yes” campaign badges during a pro-independence march in Edinburgh, Scotland, for the upcoming vote on Scotland’s independence from the United Kingdom. (Jill Lawless/AP)

With support growing for independence just months before Scots decide whether to break up with Britain, outsiders are weighing in with impassioned calls to keep the three-century-old union together.

British officials from across the political spectrum, European Union leaders, corporate executives and retired military commanders have made their cases. Even David Bowie registered a plea for Scotland to “stay with us.”

But there has been conspicuous silence from one major player with a lot to lose if Scotland secedes: the United States.

A vote for independence would dismember Washington’s closest and most important ally, leaving behind a not-so-great Britain at a moment when Russia is waging the stiffest challenge to Western authority in a generation.

Although any separation after the Sept. 18 referendum would be peaceful, it would trigger years of messy negotiations over the future of the British nuclear weapons program, North Sea oil reserves and the pound. Former NATO secretary general and British defense minister George Robertson said in a speech in Washington this past week that for the United Kingdom to “shatter this year would be cataclysmic.”

Even with such high stakes, the Obama administration has stayed on the sideline, insisting that “the future of Scotland is an internal matter.”

Now, with polls showing the unionist lead down to the single digits, the administration must choose whether to remain mum or to speak out in favor of keeping the United Kingdom together.

Analysts say a strong American statement could tilt the balance in a tight vote. But no one knows which way.

Scotland has powerful cultural and economic ties to the United States, and Obama — who is believed to have Scottish ancestry — is relatively popular there. A personal appeal from the U.S. president to the Scottish people to stick with the U.K. might cause some Scots to think twice before heading for the exits.

It could just as easily backfire.

The Scottish independence movement is built on centuries of grievance toward London, which is seen as imperious and indifferent to the welfare of Scots. As London’s closest ally, and the world’s only superpower, the United States is often viewed in the same light — as overbearing.

In general, Scots have not reacted well to the idea of outsiders telling them how to vote.

“There’s a narrative that people have of Scots as being feisty and stubborn. No one tells them what to do. And there’s a modicum of truth in that,” said John MacDonald, who directs the Scottish Global Forum, a think tank.

But MacDonald said the main reason for the backlash is that anti-independence forces are starting to panic and are trying to scare Scots into voting no.

Security implications

While a vote to break the bond with Britain was seen as a remote prospect just months ago, it has become a real possibility. And that’s deeply worrying for Western security officials.

“Nobody in the West wants this to happen,” said Phillips O’Brien, who directs the Scottish Center for War Studies at the University of Glasgow. “At some point, you have to admit you’re not a great power. And if Scotland is leaving, you’re basically left with the city-state of London.”

Scotland is just one piece of a union that also includes England, Wales and Northern Ireland. In population terms, Scotland is relatively small — only 5 million people out of 63 million.

But losing Scotland would cleave away a third of the U.K.’s landmass and a tenth of its gross national product, including a sizable chunk of the revenue from rich North Sea oil reserves. Independence could also render Britain’s nuclear weapons without a port to call home.

Nuclear-armed submarines have long been based at the Scottish port of Faslane. But Scottish leaders say they would make their newly independent country a nuclear-free zone.

In an article for the Telegraph last month, retired British Vice Adm. John McAnally asserted that Britain would have to spend billions to find a new base for its subs and could “be forced into unilateral nuclear disarmament.”

“We would be reduced to two struggling nations on Europe’s periphery, without the means to defend their now separate interests,” McAnally wrote.

Robertson, the former NATO secretary general, made a similar argument in his speech Monday at the Brookings Institution. Invoking the standoff with Russia in Ukraine, Robertson said Scottish independence would leave behind “a much diminished country” that “would rob the West of a serious partner just when solidity and cool nerves are going to be vital.”

Scottish National Party leader Alex Salmond, who is the driving force behind the referendum, responded by telling the BBC that Robertson’s comments amounted to “apocalyptic nonsense.”

Another top SNP official, Humza Yousaf, said in an interview that interventions by non-Scots “haven’t been particularly helpful to the unionist cause. I think we understand that any outside intervention is just going to work against whichever side it came from.”

The U.S. factor

Still, Scottish nationalists have been careful not to antagonize Washington. Salmond visited the United States this past week and carried with him assurances that Scotland would remain a close U.S. ally even after independence.

That is not a foregone conclusion. Scottish authorities deeply angered Washington in 2009 by releasing a Libyan national, Abdel Basset Ali al-Megrahi, who had been convicted of the 1988 bombing of a Pan Am flight over Lockerbie. At the time of his release, Scottish authorities said he was near death from cancer. But he lived three more years.

“If independence is our fate, there will be a chilly attitude toward previous Western alignments,” said Tom Gallagher, a Scottish political scientist. “The Scottish National Party is full of people who revel in doing what de Gaulle did in the 1960s — socking the Anglo-Saxons in the jaw.”

They may have even more incentive to do so if the United States comes out against independence — and loses. That result would be particularly awkward for Washington, given the United States’ own revolutionary origins.

Alistair Darling, who leads the Better Together campaign against independence, said in an interview that the United States is simply being sensitive to the right of Scots to make up their own minds.

“I quite understand why they might have taken the decision to leave this to the voters in Scotland,” Darling said.

But others who support keeping Scotland in the union say there’s room for the United States in the debate — as long as the U.S. message focuses on the virtues of saying no to independence, rather than the perils of saying yes.

“If the U.S. administration seeks to make the positive case for union, I think that’s fine,” said Menzies Campbell, a senior Scottish member of the British Parliament who strongly backs keeping the United Kingdom whole. “If it makes a case supporting the status quo based on negativity, that would be seen as interference.”

Karla Adam contributed to this report.