Bernie Sanders' quest to deal Hillary Clinton's campaign a staggering blow in next month's California primary may already be a lost cause.

A wrinkle in the rule allowing independents to participate in the Democratic presidential primary requires them to specifically request a Democratic ballot, an extra step that tens of thousands of early voters there are neglecting.

Just 14 percent of the more than 2 million registered nonpartisan absentee voters in California have received a Democratic ballot so far, according to numbers compiled by the nonpartisan research company Political Data Inc. – a glaring drop-off that observers ascribe to a pervasive lack of education about the balloting process.

Sanders' strength has been fueled by independent or nonpartisan voters, many of whom lack formal ties to a party or are completely new to the political process. Two-thirds of the total vote in California is expected to be cast by mail. But unless unaffiliated voters make a specific request to their county registrar, they will receive ballots with only local races and devoid of the top ticket choice between Sanders or Clinton.

"Half of the nonpartisan voters that are qualified say that they want to vote in the Democratic contest. Only 14 percent of them are. That's a huge gulf of hundreds of thousands of voters," says Paul Mitchell, vice president of Political Data Inc. "It's not that they didn't want it. This is just the 14 percent of the voters who successfully overcame this hurdle."

Sanders' campaign is encouraged by the influx of at least 1.3 million new voters who have registered as Democrats or independents since the beginning of the year, but they acknowledge the balloting hiccup for nonpartisan voters is of concern.

"It's a challenge, but honestly our campaign has faced far larger obstacles to people voting," says Ben Tulchin, Sanders' California-based pollster. "It's potentially going to prevent some voters from voting for Bernie who want to. With this obstacle we've got to find other opportunities. But at the end of the day, independents can still vote."

Polling of the Golden State Democratic primary has been scattershot, but Sanders is making an aggressive, full-throttled push there with daily rallies that are drawing tens of thousands of people. On Tuesday, the campaign released its first television ad in the state in over a month. Clinton's schedule hasn't been as breakneck, but the former secretary of state will spend four days in the state this week ahead of the Memorial Day weekend.

Tulchin wouldn't comment on his private polling of the race, only to say that Sanders is within "striking distance" of Clinton. But Democratic operatives with deep roots there say that given the reliance on the early vote, Sanders' efforts now may be futile.

"Even if the Sanders campaign got their act together today, it's too little, too late," says Mike Trujillo, a Los Angeles-based Democratic consultant who was Clinton's California field director during her 2008 run. "I don't think he's going to be able to turn it around to take advantage of independents voting in Democratic primaries."

Trujillo says the Sanders campaign should've devoted all its muscle and might toward educating independents on how to properly acquire a ballot from the beginning. He recalls that, like Sanders, candidate Barack Obama was gaining steam against Clinton in the closing weeks of the California primary in 2008 but was ultimately hamstrung by his team's unfamiliarity with how to mobilize nonpartisan voters.

"It's only an open primary if you open the door," he says. "The Sanders campaign hasn't taught their voters how to open the door. I would make sure the senator's entire speech was about that and to date that is not happening. The TV ads should be about that. This would've been my entire campaign."

Tulchin says the Sanders campaign conducted a significant phone and email push to their supporters ahead of Monday's voter registration deadline in the state. He's also counting on the assumption that many of the new, younger voters will show up on Election Day, allowing them to simply request a Democratic ballot in person at one of the state's 17,000 polling locations.

But if those voters previously mailed back a nonpartisan ballot, that would prevent them from participating in the presidential primary through a separate ballot. Mitchell, who has studied this issue extensively, says mail-in voters can still request a partisan ballot before May 31. They would just need to rush it back, having it postmarked by Election Day on June 7.

Compounding the situation is the existence of the American Independent Party – which 3 out of 4 Californians didn't realize is a far-right party, according to a survey by the Los Angeles Times. Whatever the intentions of the party's members, they won't be able to participate in the Democratic primary either.

Confused yet?

Many of California's own voters are.

An April survey of nonpartisan and permanent absentee voters by Mitchell found that only 43 percent could correctly identify the process for obtaining a partisan presidential ballot. Fifty-seven percent didn't think they needed to take any extra steps or were unsure.

Because California operates a "jungle primary" for down-ballot races – in which Democrats and Republicans compete against each other for the top two slots to compete in November – voters there are accustomed to seeing both parties on their primary ballots.

That's made this extra step even more unusual, according to Mitchell.

"We've trained these voters that they have all parties on their ballots and we tell them the Democratic primary is open," he says. "They've gotten very little information that they have to request a ballot. Mailing somebody a notecard doesn't really cut it."

The Sanders campaign maintains they're still in the hunt for a victory, but if the Vermont senator ultimately falls a couple points short, the reason will be clear to critics of a system that places a burden on the state's fastest-growing voting bloc.