He said it would be easier for his party to abandon Scotland, where it has just one MP, and instead aim for a majority in the parliament of the remaining part of the UK. But he insisted: "I care more about my country than my party."
Perhaps it was the corporate surroundings, the fact that bosses were watching, or a genuinely broad support for Better Together among the audience, but the nearest the crowd got to a critical question was when a woman asked whether "Westminster MPs would defer their pay rise" to keep Scotland, and requested the removal of Trident nuclear missiles.
Cameron pledged to abide by the result of the vote, even if Scotland takes the "heartbreaking" decision to vote for independence and he has to work on the disintegration of his own nation.
"We would have to make it happen," he said. "You can't hold people within the United Kingdom against their will. We in many ways are a model to the rest of the world."
Instead, he tried an emotional pitch on the brilliance of the existing union: "It works. And the whole world looks at the United Kingdom and thinks: What an extraordinary country that has done extraordinary things."
An hour earlier and just up the road, SNP leader Alex Salmond and his deputy, Nicola Sturgeon, were wandering around Edinburgh, surrounded by supporters brandishing "YES" placards.
Cameron, in contrast, was driven straight into the underground car park of the Scottish Widows building in a blacked-out vehicle, and afterwards headed straight back out to visit a local company.
"I hoped he would walk about," said a woman, a No supporter stood by the ramp as he sped past.