High voltage direct current lines can already cover vast distances. Siemens has started supplying equipment for a Chinese HVDC project that will transmit 12 gigawatts over 3300 kilometers:

https://www.siemens.com/press/pool/de/pressemitteilungen/201...

That's a long enough line to stretch between California and East Coast states.

Getting rights-of-way for big interstate transmission projects is harder than actually building the projects after those rights are secured. Superconducting cables can carry more power per cross section than HVDC lines, but they're a lot more expensive and immature. Nobody has yet found a project that would justify full-scale superconducting lines over more conventional high voltage lines built with ordinary resistive conductors.

The technology that's being deployed right now for this is high-voltage DC:

https://pv-magazine-usa.com/2018/07/09/eia-examines-hvdc-for...

It's an infrastructure investment that would pay dividends, but requires getting through a quagmire of politics.

At 3% loss per 1000 km (625 miles) HVDC is already good enough I'd say.