Editorial: Our society has a vested interest in maintaining a political system in which candidates are free to adhere to their principles.
Conclusion (Thus)
Government should pay for candidates' campaigns.
Evidence
Society has a vested interest in a political system where candidates are free to adhere to their principles.
Campaigns are so costly that most candidates must seek private funding, and in doing so they almost always have to compromise their principles.
Evaluate
The correct answer should be a principle directed at Governments whose right side is telling them what they should spend money on.
What about the trigger? Why does the author think the government should pay for campaigns?
Because in the current system, most candidates need to get private funding which almost always causes them to compromise their principles, and we have a vested interest in making sure candidates are free to adhere to their principles.
Since those two claims have an overlapping idea "the candidates' principles", we might see them Term Swap the idea of "something society has a vested interest in maintaining" into the 2nd sentence.
Goal
Look for a principle like,