The site has a protocol to evaluate candidates which determines how they can be promoted on the site. The evaluate protocol is defined here:

Candidates will be classified one of four ways:
• Mission Supporting
• Not Evaluated
• Defensive / Strategic Purpose
• Non-Supporting

Content Guidelines - Promotion Limits for non-Mission Supporting Candidates
The explanation of the candidate classes and how they can be promoted based on the classification is specified here:

Evaluation Status of 2016 Presidential Candidates

Mission Supporting
Rand Paul (campaign suspended)

Hillary Clinton - Non-Supporting based on obvious differences with our Mission.
Donald Trump - Evaluation thread

Not Evaluated
Darrel Castle - Evaluation thread
Johnson / Weld ticket - Evaluation thread
All others