In principle it's FF, but in current practice would be HP due to gerrymandering. Congress could mandate a neutral commission for CDs to eliminate that weakness. Then candidates would be seeking swing districts to compete in, not just swing states.
If POTUS were elected by CD, then the Dems would have a much stronger case, that the CD lines need to be gerrymandered in their favor, to offset the GOP's geographic advantage, given the concentration of Dems in large inner cities. If it takes say a Dem getting 53% of the popular vote, with CD lines drawn per the Muon2 metrics, in order to win a majority of the CD's (that is probably an estimate this is not very far off the mark is it?), then the system is a fail out of box.
One of the strengths of the UCC rule is that it diminishes the ability to shore up lean-R suburbs with rural areas and it prevents midsized UCCs from getting chopped to the detriment of their Dem urban cores. The SKEW for our various efforts has not shown a pronounced bias for Pubs. The biggest danger would be in those places where sufficient minorities are placed in one CD to meet the VRA and there are relatively few white Dems to provide for non-minority Dem districts. As SCOTUS moves towards sub-50% performance levels for VRA districts, this becomes less of a problem.
It's also worth noting that a decent number of swing districts produced from a muon2-styled metric would create greater responsiveness in the result to changes in electoral mood. Gerrymandered states typically have few CDs near the tossup up PVI so they are immune to small swings in voter sentiment.
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