It was early 1987, and Mitt Romney was shopping. And he was angry.

He was walking the aisles of Staples, a little-known retail store his firm had bet $1.5 million on so far, and picking up office supplies. Then he waited in line at the checkout counter — for far too long.

To Romney, then chief executive of a fledgling investment firm called Bain Capital, it confirmed what friends had told him: A superstore with low prices was a good idea, but the lines took forever, the credit card machines weren’t working, the staff was “surly.” With just a few stores open since its launch in Brighton, Staples already had big problems.

“I was shopping there myself and found it a frustrating experience,’’ Romney would later say.