Can I use a passport card as ID in a bar?

This generally varies by locality. Most places, to my knowledge, simply have laws limiting the age of persons wishing to drink. How that age is determined may be left wildly open to interpretation in some places, while other jurisdictions may require specific forms of identification.

In most places, any government issued ID will suffice from a legal standpoint, although it's often up to the particular security guard/bouncer whether he wishes to accept your ID.

I was with a Mexican friend once in the US, who was denied entry into a bar because the bouncer could not read the birthday on her Mexican-issued ID. If she had been carrying her passport, I expect she would have been allowed in.

I would be somewhat surprised if there are any laws that require that a passport (card) be considered a valid proof of age, but I would be even more surprised if there are any places that prohibit it. This means it's mostly going to be left up to the local security guards to make a judgment call.

In most places, I imagine it would be accepted.

Minnesota only accepts driver's licenses, instructional permits, or ID cards issued by a United State or Canadian Province; a US-issued military ID; or a passport book. See Subd6: https://www.revisor.mn.gov/statutes/?id=340a.503

We can't take a Puerto Rican ID of any kind (despite their territory status), a Resident Alien card (green card), an Employment Authorization Card (EAC), a Global Entry card, or a passport card. I would wager a bet that the vast majority of bartenders/servers here don't know they can't technically take those, since I've turned people down who subsequently shouted at me that "everyone else takes this, I've been served here before on this ID". It's a bit stupid that a state can decline to accept a US-government-issued ID that includes the individual's photo, birthdate, and an expiration date, but such is the law.

TL; DR: have your state-issued license or ID with you, or your full passport book, when you go to a bar or restaurant.

Actually, in California (as well as some other places) the passport ID CARD is NOT accepted as a valid form of id to purchase alcohol. This is because it lacks some of the information required, mainly " a physical description" of that person (height, weight, eye/hair color etc.). I know this may seem crazy, being that it is perfectly valid for other things, but this is the case legally. You may get away with it though if the person checking your id is careless or uninformed. Best bet is to make sure to bring another acceptable form of id.

It depends. In Massachusetts, where I live, for example, passport cards are accepted, but licenses/IDs from other states are not, which is absurd (not that many people carry passports when travelling domestically after all). However, this is often not enforced, and any reasonable ID (e.g. US state/Canadian provincial license) in practice are usually accepted.

So, the answer is: it depends on which state you are in, and which bar you are going to. I can't imagine a bar which would not accept a license or ID from the state which it's in, and it's rare to see a bar which would not accept a passport. However, in practice, most places do accept reasonable governmental photo IDs with DOB readably printed in English, so you will likely run into minimal trouble.

Dunno about the US passport card, but I've entered bars in New York and Los Angeles on a Swedish identity card (essentially a passport card) and never had a second look. Thus, it would surprise me tremendously if they were to reject the US equivalent.

But of course, it's bound to vary from establishment to establishment

It really does depend. I've used an Irish Passport Card as ID for bars and for buying alcohol in stores in cities big and small with no issue in NY, PA, IN, IL, CA, MI, RI, MA and CT however, while in Philly last summer it was confiscated by a thick headed bouncer who clearly couldn't read the word "passport" printed in bold across the top! I ended up getting it back 2 days later when a police officer had to arrest him for possessing a false identification card. Was literally the only way to get it back when the bar wouldn't even consider checking my passport book against the passport card let alone having a reasonable conversation with me! Just be careful.

The US Passport Card is a valid "REAL ID" form of identification, and thus is recognized as valid ID by all US Federal and State Government departments, in exactly the same way that a Passport is.

So officially, yes, you can use your Passport Card as valid ID in a bar within the US. There is obviously always the risk that the person checking ID's will not be familiar with a Passport Card, however they should have access to a guide that includes details on all valid ID, and the Passport Card should be included.

Outside of the US it is likely to be a bit hit-or-miss, as very few countries have any form of passport "card". Given that it does look like official US ID, it's likely it would work in many locations, but would not be as reliable as a real passport.