Millions of mail-in voters in California were sent primary ballots without Bernie's name on them. The officials in California passed a last minute rule stating voters had to make a certain specification ahead of time to vote for an independent-turned Democrat. They made sure to put it in the fine print of course, so millions of voters would pass over it, and get mail-in ballots devoid of a certain senator from Vermont's name on them.
There is a kernel of truth here, but it's a very small kernel. Most of what you said is false.
What actually happened affected only voters who were not registered as Democrats. Unlike Republicans, the Democrats in California allow people not registered with any party to vote in their party's primary.
But if you are a No Party Preference voter, you have to request a Democratic party ballot. Otherwise, nobody knows that you want to vote in the Democratic primary.
Some people screwed up and didn't request a Democratic ballot. As a result, they got a ballot that didn't list Bernie. It also didn't list Clinton.
Also, some people registered as Independent, thinking 'well, Bernie is an Independent'. But that gave them a ballot for the American Independent Party, whose candidate(s) didn't include Bernie or Hillary.
There certainly weren't millions of such cases. Thousands, maybe. The affected people didn't vote for Hillary or Bernie.
A final note is that the California primary was in June, after the race was already settled.
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