Field is a respected California pollster. They had Jerry Brown up 20 in June 2014 and 16 in August. He won by 20. They had Barack Obama up by 24 in September 2012. He won by 23. They had Dianne Feinstein up by 26 in September 2012. She won by 25. So that's a pretty good history in statewide races.
That said, those were easy races to poll. The Democrats were all popular and the Republicans had no appeal beyond Republican leaning voters. This race, however, is different than any we've ever had. It's a race between two Democrats and no Republicans. Can you make sure you actually get the right mix of Republican voters who will vote and those that won't? Unfortunately, the Presidential race doesn't give you a good control group. You could've looked at the sample in 2012 and judged what Romney voters were saying they'd do. Donald Trump isn't your standard Republican. He'll lose some voters who would've voted for Kasich, for example, to Clinton, some to Johnson, and some at home. But Trump could also get some Obama voters that other Republicans might not get.
Field shows Kamala Harris with a 39%-24% lead. A poll with 39% of the voters undecided is unusual but Field includes voters who say they won't vote in this race. Among voters who say they'll vote Harris' lead is 46%-28%. Voters who leave blank ballots aren't counted.
Unsurprisingly, Harris leads 48%-27% among Democrats. Despite both candidates being Democrats, Harris' progressive politics plays better with traditional Democratic voters. Harris does very well with Black voters. They tend to be among the most liberal. For Sanchez to win she needs to win Republican and independent voters by good margins and she also needs strong turn-out with both groups. The primary was extremely Democrat heavy. I don't think the general election will be nearly as heavy with Democrats, but it could be heavier than past general elections.
Sanchez also needs Republican and right leaning voters to vote in the senate race. In 2012 the Presidential to congressional undervote was 6% in districts with a Republican and Democrats but around 11% in districts with two Democrats. These districts, however, had fewer Republicans than the state as a whole. If Republicans decline to vote in the same proportions you're talking about a 15% undervote. The survey has a 15% undervote.
Sanchez should also be concerned that she trails 28%-16% among Republicans who've made up their minds. She needs to win Republicans at least 60%-40% and is losing them by 64%-36%. So getting Republicans to vote is no guarantee for Sanchez. Why would Republicans be favoring the progressive Harris? I'm speculating but part of it is name recognition. Harris is more well-known. Part of it might be her office. Harris has championed progressive causes as Attorney General, but many people don't know that. Attorney General sounds like a law and order position that Republicans might favor.
California is a left leaning state but the middle of the electorate is just a bit left of center, nowhere as far left as Harris is on issues. That's the opportunity for Sanchez. What she has to do is position herself as the only candidate who can appeal to the center and the right while not alienating Democratic voters that she'll need. She doesn't need to win Democrats to win this election but she needs to get a good share. How do you position yourself as two different things for two different groups? That's tricky but there have been enough Democrat-Democrat races that there is a path to victory. Eric Swalwell beat Pete Stark as the more rational Democrat in the CA-15 race. While Ro Khanna fell 4% shy of Mike Honda in 2014, he must've done something right to get it so close. Khanna beat Honda in the June primary this year, so he's likely figured out how to appeal to those voters a bit better.