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.
It's worth noting that this system would destroy any chance that local issues would predominate over national issues in the swing districts. Imagine the difficulty of convincing voters that they should focus on how you'll help the district when the area is heavily targeted by both Presidential candidates with frequent visits and negative ad carpetbombing.
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