This is one of those difficult-to-admit cases where the French were correct and the US (although a Canadian hotel chain in this case) had it wrong. Champagne is Champagne. All the rest is not Champagne.

AA used to advertise their rotgut as Champagne. It's now correctly identified as sparkling wine.

I've had some spectacular fights at restaurants when a bottle of the California knock-off (often owned by and with a copycat name of the French vintner) is brought although the menu claims that I am buying Champagne. One time the waiter had pre-opened the plonk, so that went down the drain, and the restaurant had to eat the price differential on the genuine article.