Are there any issues with speaking Spanish as a tourist in Portugal?

I was recently in Lisbon with a friend who was born and partly raised in Latin America. He generally addressed waiters and the like in Spanish; they generally responded in English. (Everyone else in or group was speaking English, and we spoke English with each other.) Nobody seemed to take offense.

Common sense seems to suggest that one offer to speak whichever language the other prefers.

I speak some Spanish, and when I was in Porto, was trying to find a particular shop. We came across some policemen, and from past experience I tend to be a little wary of offending or drawing attention to myself around them. However, they could see we were retracting steps and came up, friendly, speaking in Portuguese.

While they're similar, I can't really follow Portuguese, and tried, as well as hand gestures, but failed. I then asked in Portuguese if he spoke English, and he looked a bit embarrassed and said no. So I tried asking if he spoke Spanish, and he said 'a little'. Perfect! We conversed a few lines, got some directions, and both parties were happy - we got help, and he helped someone.

Now the key here is probably the initial attempt to speak in the local language first. Trying to work with that doesn't immediately put them on the back foot. You're in their home.

Failing that, I tried to keep some comfort - I speak English natively, and it's a fairly common second language so asked about that.

THEN, I tried to look for common ground. (Hilariously this failed in Belgium where a shop owner and I tried in 6 different languages, each time hitting one that the other person didn't know). This is when it's suitable to do.

If you wanted a shorter, way, learning to say "I'm sorry, my Portuguese is poor, do you perhaps speak any English or Spanish?" would not be too much of a stretch, and is still polite to ask in their language first.

To the heart of your question - are there issues? Yes, not everyone will understand you, and secondly, many people feel uncomfortable when not addressed in their natural language, they're on the back foot.

This is my experience from five days in Porto last week:

Portuguese is very similar to Spanish and reasonably similar to other Romance languages. With a good level of Spanish, you should get a good grasp of Portuguese, and nearly all Portuguese people we met seemed to understand Spanish. Probably the fact that there is a lot of Spanish tourists in Porto has given everybody in any public facing position some exposure to Spanish and even supermarket employees in not very touristic areas understand some Spanish.

On the other hand, most people answered to us in Portuguese and that lead to some reasonably efficient bilingual conversations. I'd say they didn't speak fast to us, and maybe they tended to use a simple Portuguese.

I agree that trying to speak the local language is the best thing to do as a tourist and it's part of the fun and interest of travel. However, I must admit that I couldn't find any practical difference on the reactions to my intents at Portuguese (actually a mix of Portuguese, Spanish, Catalan and imagination) compared to my spouse's ones in plain Spanish. I even recall one instance when my carefully crafted Portuguese query at the ticket office in Sao Bento station ("Quatro pessoas a Guimarães") was simply answered in Spanish ("Ida y vuelta?") to my disappointment.

In the end, I'd say the people we met were very kind - some of them, kind far beyond their expected professional kindness - and that eases understanding a lot, in addition to similarity of languages.