So, Greg asks a great question:
Why isn't there more state-level gerrymandering? That is, when there's unified partisan control of the federal government, why don't parties use that to admit more states to bolster their advantage in the Senate? If New York City (or even each borough) seceded from the state, there'd be two strongly Democratic states instead of one. Texas could split off several chunks to create more Republican states.Two answers. First is: there are all kinds of institutional crosscurrents involved that make it harder than it might seem. Politicians aren't only party actors; they also have their own personal self-interest, which often conflicts with a party interest...for example, a New York Democratic Senator from upstate might worry that she would put her seat in jeopardy without the NYC vote, and prefer two safe NY seats instead of two extremely safe NYC seats plus two lean-Dem other seats. At the same time, some of her constituents may have strong interests in keeping the status quo in New York state government. The bottom line is "what's good for the party?" is often not the most important question politicians and other party actors ask. I'll point out, too, that it would surely have been filibuster bait any time in the last thirty years at least, and maybe even before that, and there haven't been too many occasions to test it in that case.
Second answer? I don't know! It's a complete mystery to me why Democrats in 1993 didn't try to make the District into a state, and even more of a mystery why Democrats in 2009 didn't do it. I've heard several suggested answers (basically they come down to what I think is an absolutely wrong belief that anyone in Nebraska or Missouri would care in an election two or four years later, although it's certainly plausible that the Benator or whoever actually believed it, even though it wouldn't have been true).