Republicans would campaign in NY and California. There are sizeable Republicans in those states (as a number, not as a percentage of the state’s electorate). So increase the vote total in those states, hold on the currently Republican states, and they have a shot.
It’s speculative to think that Republicans wouldn’t win. If they knew they needed the popular vote, then they would campaign differently than they do now.
I agree with this for 2016. Most of the 2016 3rd party vote was for right-leaning candidates and 3rd parties ran better in non-competitive states. Combine that with Trump making 0 effort in California and Clinton putting effort into some of the wrong places for the EC, and Clinton falling short of a majority in the PV, and it becomes very plausible Trump could win an election contested on NPV rules. Because of the 3rd party vote, I would go so far as to say Trump would be favored if the NPV election had a runoff round.
However, Bush's win in 2000 IMO can only be explained as a fluke of the EC and plurality wins. Bush was already from Texas and he campaigned actively in California, so unlike Trump I don't see how he squeezes more votes out of either of those states? And if there's a runoff round it's abundantly clear Gore picks up a lot of the Nader vote. A runoff result could easily be 52 Gore/48 Bush.
