With the California primary just days away, Democratic pollster Doug Schoen writes in The Wall Street Journal that the party's struggling front-runner Hillary Clinton could be denied the nomination if Bernie Sanders wins the Golden State race on June 7.According to a recent PPIC poll Clinton still leads the Vermont Senator by 2 percent, however Schoen notes that even a narrow win would give Sanders a significant boost in pledged delegates — 250 or more.And, in recent open primaries, Schoen backs his statement by noting that Sanders has a tendency to "underperform in pre-election surveys and over-perform on primary and caucus days, thanks to the participation of new registrants and young voters."A loss for Clinton in California would not only "powerfully underscore" her weakness as a candidate, but make the Democratic superdelgates — who currently support Clinton over Sanders by an overwhelming 543-44 — question their backing and change their opinion.Although Sanders has consistently won delegates in the ballot box, the Vermont senator continues to lose to Clinton because of her significant superdelegate lead. However, Schoen writes that Sanders could potentially change convention rules "requiring superdelegates to vote for the candidate who won their state’s primary or caucus."Schoen continued, "A vote on that proposed change would almost certainly occur —and it would function as a referendum on the Clinton candidacy."In addition, after the State Department inspector general's recent report showed Clinton breaking rules and being dishonest in her statements regarding her use of a private email server while she was secretary of state, she faces an upsurge in legal problems — further damaging her presidential campaign."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 writes.And although Joe Biden decided against running for president this season as he's still mourning the death of his son, Beau, recent reports have noted the vice president expressing deep regret on his decision of not entering the race.While Clinton's negative ratings are just as high as Donald Trump, Biden may be eager to enter the fray, Schoen writes."Mr. Biden would be cast as the white knight rescuing the party, and the nation, from a possible Trump presidency," the Democratic pollster said."To win over Sanders supporters, he would likely choose as his running mate someone like Sen. Elizabeth Warren who is respected by the party’s left wing."With all of these possibilities, Schoen, who served as a political adviser and pollster for President Bill Clinton from 1994 to 2000, says, "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."