On the eve of the Scottish independence referendum, President Obama tweeted his hope that the UK will remains "strong, robust and united."

The UK is an extraordinary partner for America and a force for good in an unstable world. I hope it remains strong, robust and united. -bo — The White House (@WhiteHouse) September 17, 2014

This isn't the first time Obama has made his support for the No campaign in Scotland explicit; he made similar statements alongside Prime Minister David Cameron at the G7 meeting in June. But that was before a series of polls suggesting the Yes campaign was within the margin of error for victory.

Scotland's separation would throw the UK's military and defense into turmoil, and weaken its participation in military actions against the Islamic State or in Ukraine, according to Foreign Policy.

Former President Bill Clinton and former Secretary of State Hillary Clinton also oppose Scottish independence. The former president released a statement Wednesday saying "it is possible to respect our differences while living and working together,” while Hillary told Jeremy Paxman in June that a Yes vote would be "a loss for both sides".

The most recent polling suggests Scotland will vote to stay in the union by a narrow margin. If undecided voters lean towards independence, the nationalists could still easily win.

If that happens on Thursday, Scotland will break its 307-year union with England and become an entirely separate country. They'd gain self-governance and oil revenues, but could lose the pound and the BBC.

Obama's tweet didn't deter Scottish supporters, who chanted his famous 2008 slogan "Yes we can!" at a pro-independence rally Wednesday.