See the OP.
It is the idea that folks have to vote for her when they feel she hasn't earned their vote.
"Fall in line" is basically the worst thing you can say to convince a Sanders supporter. His entire career has been about fighting for what he thinks is right.
There is a difference between compromise and compromising your principles.
This strikes me as a misunderstanding of the argument. Although, admittedly, the OP is a terrible standard bearer for it since it's just a pile of undifferentiated rage.
I think a certain amount of disconnect here is just about what voting is and what it means. The argument most Hillary supporters are putting forward is based on a model of voting which is something like:
* All candidates put forward a policy platform
* All voters weigh their policy preferences and the perceived likelihood of each candidate achieving their policy goals and rank the policy platforms in order of preference (if you only have policy preferences that don't appear on any platform, you have to do something like rank the platforms based on which one is most likely to create future platforms that do contain your preferences)
* Voters then vote for the candidate with the policy platform they prefer the most or disprefer the least, factoring in success probability
* The candidate that is elected then governs more or less towards that policy platform
In this model your vote is primarily a small opportunity for you to control the policy outcomes of the country. Note that in this model any third-party vote or absent vote is more or less totally irrational because the third-party candidates have approximately zero probability of winning and enacting their policy goals.*
So most of their arguments are basically "if you prefer Bernie's policy platform to Hillary's, then your demonstrated preferences are such that you are likely to prefer Hillary's policy platform to Trump's; therefore doing anything other than voting for Hillary is irrational."
I think this is fine as far as it goes, minus the, you know, rage. But this is not necessarily the election model that a lot of people have, and I think that's creating disconnect.
For example, when I hear people say "why should I vote for a candidate I don't like" it suggests to me an election model that goes something like:
* Each candidate makes an appeal to voters to convince them that they are the best person to run the country
* Each voter chooses whichever candidate has convinced them, or no candidate if nobody has convinced them
In this model your vote is a personal stamp of approval rather than an exercise of your sovereign franchise.
This is probably a pretty common model among a lot of voters, and not an unreasonable one to have -- it sounds a lot like what you would expect a democracy to be. Obviously there are a lot of people who don't agree with it and think it is "not tactical." But arguments based on Hillary being the rational choice don't do a lot for people who are concerned with personal appeal. If you were convinced that Bernie Sanders would do a good job of running the country, and you are not convinced that Hillary Clinton would do a good job, then the argument that her policy platform is preferable to Donald's seems somewhat out of the blue.
There are a lot of other difficulties along these lines as well. For example, there are a few posters who have said something like "Donald Trump has said he will do policy X, but I don't believe him, I think he will do policy Y." These people seem to be applying more or less the rational policy preference model; however, their approach to identifying a candidate's platform is nonstandard for some reason. That is a totally different argument to have!
Anyway, that is more or less why I think these threads go so poorly (along with, like, the fact that this is a dumb point in time to be arguing with Sanders supporters that they should support Hillary). It's not necessarily about the candidates or the policy every time. Sometimes it's just about how we understand democracy and franchise to work.
* I said more or less! I think, for example, the argument that voting Green in California is a mechanism for convincing the Democratic Party to be more liberal is pretty solid and defensible.
