My biggest question is who will give in, Europe or the US? If I recall, the US keeps good records of the percent of people from each country that commit crime, overstay their visa, etc. There has been some kerfuffle that we require citizens of one of the US' best friends, Israel, visas. But the US backs up that requirement showing the percentage of Israelis in America engaged in various activities. I'd imagine the same is similair for Romania / Bulgaria. So the US has good reasons.

At the same time, Europeans have a lot of pride and don't like thinking America controls them.

My uneducated guess: the Europeans strong need to not be controlled by America will mean that Europe will either implement, or give one last ultimatum and then implement.

US citizens will start calling their congress people when they realize they need a visa for their trip to Rome. I'd guess the congress people will give in and make the change, as there aren't so many Bulgarian / Romanian people causing trouble compared with angry travelers wanting to travel.

If the rule does get implemented, then the question would be, who is the greater political force: the European tourism industry losing revenue or the American traveling citizenry dealing with hassles?