A former political adviser and pollster to former President Bill Clinton says there's a chance Hillary Clinton won't win the Democratic nomination for president.

In an op-ed for The Wall Street Journal, Douglas Schoen laid out possible scenarios.

"A Sanders win in California would powerfully underscore Mrs. Clinton's weakness as a candidate in the general election. Democratic superdelegates--chosen by the party establishment and overwhelmingly backing Mrs. Clinton, 543-44--would seriously question whether they should continue to stand behind her candidacy," he wrote.

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California is among six states holding primary contests on Tuesday and the Golden State has 475 delegates up for grabs.

Schoen argues that Sanders might offer a rules change at the Democratic National Convention that would require superdelegates to vote for the candidate who won their state's primary or caucus.

"If Mr. Sanders wins California, Montana and North Dakota on Tuesday and stays competitive in New Jersey, he could well be within 200 pledged delegates of Mrs. Clinton, making a vote in favor of the rules change on superdelegates more likely," Schoen said. In 2008, the gap separating Clinton from then-Sen. Obama was 102 pledged delegates -- about half of Clinton's lead over Sanders.

Schoen also said that Clinton's argument that she would be a strong candidate to beat Donald Trump in November doesn't reflect recent polls that show the race could be very close.

He also says that Clinton "faces growing legal problems" over his use of a private email server when she served as secretary of State. Clinton has defended her use of it, explaining that the government changed the rules after she left office. She has also repeatedly said that it was a mistake.

Schoen wrote that a messy convention in July could also hurt Clinton's chances and that another person like John Kerry or Joe Biden could emerge as possible candidates, which is incredibly unlikely.

"All of these remain merely possibilities. But it is easier now than ever to imagine a scenario in which Hillary Clinton--whether by dint of legal or political circumstances--is not the Democratic presidential nominee," Schoen said.

Schoen's theory is unlikely, however, because Clinton is not far away from the required 2,383 delegates needed to win the nomination. She needs just 71 more pledged delegates and superdelegates to clinch it.