Are US dollars accepted in Portugal?

I'm curious, do they accept US dollars in Portugal? I'm planning a trip there and want to make sure I'm prepared with the right currency. Is it easy to find places that will accept American dollars, or should I focus on exchanging my money for euros before I go? I've heard that some countries prefer the local currency, so I'm wondering if Portugal falls into that category. Any advice or insight you can provide would be greatly appreciated.