Vince Vasquez, a senior policy analyst at the National University System Institute for Policy Research, said he believes "there's a good chance for a Sanders upset" in California, which holds its primaries June 7.
"We may actually be witnessing the same electoral conditions here that occurred in Michigan, where Sanders had a surprise victory on election night, mostly due to spotty polling, pollsters underestimating the millennial vote and Hillary Clinton underestimating the appeal of Sanders," Vasquez told ABC News.
A loss for Clinton there would be "in a word, embarrassing," Vasquez said.
The state's delegates are divided proportionally, and Clinton and Sanders are expected to have a close finish, so it is unlikely that a Sanders win would prevent Clinton from securing the number of delegates she needs to clinch the nomination next week. Most likely, both candidates will come out of California with three-figure delegate hauls.
California is one of six states that hold their primaries next week, and Clinton needs only 73 delegates to clinch the Democratic nomination. Given that voting in New Jersey, a state that will be awarding 142 Democratic delegates, close first, there's a chance she will clear the threshold before polls in California close.
Still, regardless of the outcome, Sanders may very well have already changed the expectations for Democratic candidates in the Golden State. He has been taking a retail politics approach to California, even though it’s the most populous state in the country. So far, he has held over 20 rallies there, where he has spoken to over 130,000 people, according to his campaign’s estimates. He previously said his goal was to have 250,000 people attend his events before the primary.
"I’m not sure that has been done in recent history, and the reason we’re doing that is I believe in grass-roots politics," Sanders said Monday in Oakland. "I believe that the people in California and other places have the right to see the candidate up front, ask questions with a lot of town meetings as well and find out where he or she is coming from," he told reporters over the weekend.
Vasquez said that Sanders' campaigning schedule has helped him get more media attention beyond the $1.5 million ad buy he announced last week.
"Sanders has been campaigning aggressively here in California with large public rallies. That’s kept him in the local news cycle and generated millions of dollars’ worth of earned media," Vasquez said.
Positive Inroads for the Party
"We’ve had a historic surge in new voter registration since Jan. 1. Estimates are around 2 million new voters were added to the rolls," Vasquez said of California. "That includes both brand-new voters and those that are reregistering. A large chunk of those voters are millennials, Democratic-leaning and overall fit the demographic profile of Bernie Sanders supporters."
Clinton has made changes to her campaign schedule in recent days, adding an event in Oakland on Friday and canceling a New Jersey event this coming Thursday in favor of going to California earlier.
The schedule changes come as polling suggests the race in the Golden State has been tightening. The Public Policy Institute of California released a poll last week showing that Clinton was leading Sanders by 2 percentage points, within the poll's the margin of error. The race has grown closer since the group's previous poll, released March 24, which had Clinton with a 7-point lead.
A Clinton campaign aide noted that Clinton has won seven of the last 11 primaries and that then–Sen. Barack Obama won only three of the final 10 primaries in 2008 — and lost California — but went on to win the nomination handily. That year, however, the most populous state in the union voted in February, when the nomination was still up in the air.
For his part, Sanders said he's "feeling good" about the Golden State. "We have excellent chance to win here in California, and I believe that we have a chance to perhaps win big."
Message and Momentum
Bruce Cain, a professor of political science at Stanford University, told ABC News that “it’s not about winning the nomination anymore” for Sanders and his supporters but about gaining momentum to help push his policies when it comes time for the convention.
"They want to move the party from its likely centrist direction ... They want to do what they can to prevent [Clinton] from her moving into the middle," Cain said of Sanders camp’s likely motives.
For Clinton, Cain says, it’s more about her perception heading into the general election contest because “between New Jersey and California, she’s going to be way over what she needs in the delegate count.”
"It means in the short run, there will be continued angst about her performance as a candidate and endless second-guessing as to whether the party’s made a mistake, by some anyway," Cain said.
When it comes to the general election, Donald Trump may be the biggest factor in helping Clinton and Sanders supporters resolve their differences.
"Time is on Hillary's side, and Trump is on Hillary's side, in the sense that his continued controversial statements and policy pronouncements will ultimately help to unite the Democratic Party because in politics, opposition to the other side is a far more powerful force than agreement," Cain said.
ABC News’ Liz Kreutz contributed to this story.