US Constitution, Article IV, Section 3: New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress
So West Virginia is not a state?
As for West Virginia's entry into the Union, this was essentially a extra-constituitonal wartime measure as the Old Dominion certainly never consented to the separation of the northwestern counties. Northern West Virginia was largely settled by Pennsylvanians, with economic and cultural ties closer to Pittsburgh than to Richmond. The rest of the Mountain State, though settled by Virginians or directly from Scotland and Northern Ireland, resented the Tidewater and Piedmont aristocracy dominating politics in Richmond.
Splitting New York (or California, Illinois, Pennsylvania, Washington, or Oregon) would be difficult. First, there is the question as to where the state line would be drawn. The street systems of New York City, etc., flow directly into the suburbs. Suburban areas, especially those closer to the city line, tend to be culturally and politically similar to their central city. What does Great Neck have in common with the Finger Lakes, Pasadena with the San Joaquin Valley, Evanston with Little Egypt, etc.? Then, there is the question of where suburbia ends and whether outer suburbs or exurbs would prefer to align with the new state or the original one. Would, say, Irvine or San Juan Capistrano really want to join a liberal dominated coastal state or remain under the authority of Sacramento?
State splitting would be a good thing for Republicans and conservatives. Presently, the six states I mentioned above are represented by only one conservative Republican (Santorum), one moderate Republican (Gordon Smith), one RINO (Specter), and nine liberal and leftist Democrats (Boxer, Feinstein, Durbin, Obama, Hillary Clinton, Schumer, Patsy Murray, Cantwell, Wyden). Basically, we are looking at 1 1/2 votes for the good guys (half credit for moderate Republicans) to 10 1/2 for the libs, or a 12.5% ratio. Were 12 states to replace the existing six, the city-based states would vote 12 liberals into the Senate with little doubt. Of the rural based states, all but downstate Illinois and upstate New York would be solidly GOP. Assuming a 50-50 conservative moderate GOP split, the "rumps" of OR, WA, CA, and PA would give us four conservatives and four moderates. My guess is that downstate IL would give us one conservative Republican and one moderately liberal Democrat, while upstate NY would give us a moderate Republican and one liberal Democrat. The final lineup would be five conservative Republicans, five moderate Republicans, and 14 liberal Democrats. The good guys would increase their percentage to 31%.
No doubt the Democrats can do the math as well and would oppose any state splitting on both the state and Federal levels.