I'm not an expert, but just for now I'm going to play one on Daily Kos:
Speaking as a citizen who has helped, over the years, to count the ballots that are cast in my town for various elections [it's a dirty job but somebody's gotta do it and it only takes one or two dozen people a few hours, or deep into the night for the exciting ones, but for me actively helping makes the time go by faster than just waiting by the tv while idiots speculate] -in NH...
One Thing I Know: Write-ins Happen.
We get people wasting their vote on Mickey Mouse and Donald Duck- those are counted as "other" in order to keep track of the exact tally of the number of ballots given out vs. the exact number counted altogether.
We get people writing in themselves or their friends, and when that includes the small town electable jobs like cemetery trustee, that sometimes makes a difference. And it is true that the votes technically count only within their parties but here we are not usually talking about contested races, in fact we get a lot of ballots left blank, which again are tallied to keep track of total numbers. And cross-party voting doesn't usually come up as an issue in these races, although it technically could. The thing I have never ever heard, though is-
'nope, You can't write that other-party guy in on this party's ticket..' because, as far as I know, you CAN. And the vote counts. On the ballot of the party you wrote it on.
We get Republicans who really don't like their own choices, who write-in one of the Democrats on their own ticket in the primary. This type of vote literally counts as a vote for The-named-Democrat to be the Republican party candidate, though, because Republicans and any Independents who want to join them for the day can vote only for how to shape the republican ticket that way.
Similarly, anyone who is a registered Democrat, or an Independent who declares upon entering- [when they smile and ask 'which way today?' since there can be various local reasons for switching and people do it all.the.time] -can then proceed to write-in Bernie Sanders, or Mickey Mouse, or even Ted Cruz if they so desire out of the weirdness of their hearts. It won't help Ted- it will only give him one silly little vote to be the Democratic nominee, this is not mix-n-match on the ballot. Once you declare your allegiance for that primary election, you are choosing with-in that party. But that also means that it is as simple as that:
If everyone in NH who wants Bernie Sanders for President is willing to register as a Democrat, or REGISTER EARLY AS AN INDEPENDENT (you MUST do this if you are a Republican who would rather place a vote for Sanders that will count in the Democrats' primary), and then change your registration on that day to the Democratic Party just for the Primary [February 9th, 2016 mark your calendars now!] then we are all eligible to write
"BERNIE SANDERS" IN THE WRITE-IN SPOT ON THE DEMOCRAT'S PRIMARY BALLOT.
They cannot stop us from doing that, and they cannot question it as far as I know if we do it either. If this is not correct, I humbly apologize, but FWIW, AFAIK, as they say...
Don't let them scare you into NOT voting for Sanders because we're NOT supposed to or 'we can't'.
One of the first things I heard Sanders say in person, when confronted with the idea that he 'won't be able to get anything done with a difficult Congress' (as if they aren't actively blockading his bills in the Senate now), was that the revolution will require a groundswell of people filling up the lower offices, and that type of attitude will also elect him.
For years I have written-in Kucinich rather than take what was offered. This time around I remember joking over a year ago, saying, 'Hey Democatz- I'z in yer primary, votin' for Bernie Sanderz' -and I was thrilled when Bernie came along with Plan B: 'I'll Be a Democrat' - but back to plan A- 'add him to their ballot' is still fine with me 'cause it's not really very different and doesn't take any longer.
Presidential Candidate Bernie Sanders says: "You'll have to do it yourselves."
Nuclear Winter Solstice sez: Don't let them stop you.