Huh? Which pollster is claiming that? This article from 3 days ago seems to indicate the opposite - Harris is highly unlikely to lose the popular vote but the EC is very much still up in the air.
California has a lot of people who vote, it’s tough to make up that margin.
National polling isn’t the same as a poll designed to predict the popular vote. Biden won in 2020 by 7 MILLION votes. Even Clinton beat Trump by nearly 3 million votes. The popular vote and the election results are decided by completely different factors, pollsters aren’t soliciting people outside of swing states right now because nobody gives a fuck what a random Californian or Missourian thinks at the moment.