Just looked it up. There were 32 cities (not counting Washington DC) that had over 500,000 people as of the 2010 census. That would mean 32 brand new deep blue states, out of a new total of 82 states. Your proposal would leave states such as Maine, Vermont, Connecticut, Rhode Island, New Jersey, Delaware, Minnesota, Iowa, and Hawaii unchanged. That’s 9 more states that are deep blue, for a total of 41 states, or half of the states. You are going to get at most a handful of GOP Senate seats out of those states. You would have to almost sweep the rest of the states for the GOP to control the Senate. Sure, states like Michigan, Oregon, Illinois, Pennsylvania and New York would become more conservative, but it’s still a very tall order.
I realize they’d be deep blue - but they would free up the other parts of the states - if the number needs to be 1,000,000 I’m ok - then it’s only like 9-10. Maybe 800k for top 12 that would free the rest of Illinois, New York, Michigan, California, Penn. 3 of top 12 are Texas, 4 of top 12 are Cali...though I imagine SD being a conservative leaner and San Jose a toss up maybe, while LA and SF would lean hard left. 3 in Texas might split.