A former political adviser to President Bill Clinton says there is "more than a theoretical chance" that Hillary Clinton will not be the Democratic nominee for president.

Douglas Schoen wrote in the Wall Street Journal late Tuesday that a Bernie Sanders win in California next week would remove the "inevitability" of Clinton's campaign. That could force Democrats to realize that Clinton might be the weaker candidate in a general election.

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

Schoen said Sanders is also likely to use the Democratic convention to push for a rule that would force super delegates to vote for the candidate that won their respective states.

He said that proposal could end up being a "referendum on the Clinton candidacy," and would give Democrats a way to hand the nomination to Sanders.

Schoen said these sorts of issues are going through the minds of Democrats in the wake of polls showing that Sanders would perform better against Donald Trump.

"Bernie Sanders consistently runs stronger than she does against Mr. Trump nationally, beating him by about 10 points in a number of recent surveys," he wrote.

Another unknown factor for Democrats is Clinton's email scandal, which continues to dog her campaign. A government watchdog report last week said Clinton broke the rules by keeping her private email system, and many voters believe Clinton acted in an unethical way by not using a government email account.

"The damning findings buttressed concerns within the party that Mrs. Clinton and her aides may not get through the government's investigation without a finding of culpability somewhere," Schoen wrote.