Privacy
The government is asking Apple to hack our own users...
[T]his demand would undermine the very freedoms and liberty our government is meant to protect.
- Tim Cook, Apple CEO
Apple refused the order by the government to create a "backdoor", claiming that such a device would make it possible to unlock any phone. This could be used by any to access anyone's private information. Tim Cook in an appeal to it's users wrote an open letter, arguing this precedent could lead the government to access "...financial data, track your location, or even access your phone's microphone or camera without your knowledge". [1]

What Happened

On December 2nd, 2015, two Americans attacked a Department of Public Health party, killing 14 and injuring 22 people. This led to an investigation led by the FBI. However, the FBI wasn't able to decrypt one of the terrorists' phones, and in trying to access information, ordered Apple to provide a way to unlock the phone. Eventually, the FBI resolved this by finding a way to access the information, but this question of responsibility is unresolved. [2]
The government argues that Apple is avoiding helping the government access key information to protect itself. In a statement, the government said "Apple's rhetoric is not only false, but also corrosive of the very institutions that are best able to safeguard our liberty and our rights: the courts, the Fourth Amendment, longstanding precedent and venerable laws, and the democratically elected branches of government". [3]
Candidates

Bernie Sanders

"I am very fearful in America about Big Brother. And that means not only the federal government getting into your emails or knowing what books you're taking out of the library, or private corporations knowing everything there is to know about you in terms of your health records, your banking records, your consumer practices"

"On the other hand, what I also worry about is the possibility of another terrorist attack against our country. And frankly, I think there is a middle ground that can be reached." [4]

Donald Trump

"I agree 100% with the courts. In that case, we should open it up. I think security overall - we have to open it up. And we have to use our heads. We have to use common sense."

"Apple - this is one case and this is a case that certainly we should be able to get into the phone. And we should find out what happened, why it happened, and maybe there's other people involved. And we have to do that." [5]

Hillary Clinton

"I see both sides, and I think most citizens see both sides. We don't want privacy and encryption destroyed, and we want to catch and make sure there's nobody else out there whose information is on the cell phone of that killer."

"I'm ... really making a plea that the government and the tech companies keep working together to see that there isn't some legitimate way to help deal with these kinds of very real world problems that we face." [6]