If you mean the Veneto, you'd have to ask a Venetian. My "guess" is that they wouldn't give up their "language", as they insist on calling it.
For Lombardia and Piemonte, I have no idea how it would have actually worked. Half of the people in the big cities of those two provinces, i.e. Milano and Torino, are of southern Italian extraction. It always seems that all the taxi drivers in Milano are Calabresi who start trying to talk to my husband in "their" language. In Genova and even La Spezia the same thing has happened. In La Spezia they tried to talk to him in Neapolitan. One good thing is that you can now get great Southern Italian pastry even in La Spezia, and at our big food festivals, you can get Southern Italian food as well as ceci, and farinata etc.
Maybe Stuvane will chime in and give his perspective on Lombardia and Romagna.
I've lived my adult life here in the U.S. so a lot of this comes from reading and watching Italian news and from conversations with my own family. Even with them I have to be careful of expressing myself plainly. As I've been told, if I don't pay taxes there, my opinions shouldn't hold as much weight. Fair enough, I think. The same goes for the fact that my son will never have to put his life on the line for Italy, although I would. I'm not one of those people who think Italians who have citizenship elsewhere, especially if they're second and third generation, should be voting in Italian elections. You should have skin in the game imo.
That said, and since my relatives aren't reading this, I think it was all a crock of you know what. Not that most of my family is in Lega Nord. Most members of my maternal family were or descend from partisans of the mostly Socialist and Anarchist kind and they still vote that way. My paternal grandfather, had he been alive, would certainly have joined Lega Nord, and joined in the spray painting of "tags" telling the "terroni" to go home, no doubt. (I lied through my teeth about that to my husband, I have to admit, or I would never have gotten him back to northern Italy to visit. Mea Culpa.) My father told me more than once that had his father been alive there would have been real trouble when I got engaged to my husband. Not that I would have cared. I really didn't like him very much at all, truth be told. He wasn't a drunk or violent or anything like that, but a more obnoxious, more selfish, and at the same time more incapable man it's difficult to imagine; he never made a good decision in his life after the one where he married my grandmother. Just suffice it to say that everything my father and my aunts and uncles became was thanks to my grandmother. She has undoubtedly been sainted for having put up with him for more than fifty years.
The whole Lega Nord thing was just all completely impractical, which is why they've given it up: too many southerners in the north; too much admixture; too much change in the language in most places etc. Plus, they quickly realized Switzerland or wherever was certainly not going to accept them in some sort of "Confederation of Alpine Peoples".

Switzerland, for goodness' sakes, the only place where I ever in my entire life experienced anti-Italian bigotry. I've never been back since.
Now all they care about is taxes and a federal system. Right, they want to keep all their income. OK. A really regional way of doing this didn't work out so well for them in this Covid Crisis.
Well, that's my rant. Other Italians, at least the ones who still do have skin in the game, can tell me I'm wrong if they want.
What a joke that whole "terroni" thing about southerners is, anyway. Who the hell could be more "terroni" than the people of the Po Valley, and yes, I mean north of the Po Valley and into the Alps too. They were dying of pellagra in the 1900s and sending their daughters to be serfs in the rice paddies in the beginning of the 20th century. Why else would so many of them gone to die in the jungles of South America? People don't leave en masse where they're eating well.