Conversation so far:
Who was the Republican nominee relative to Ron Paul?
Ron Paul said that under no circumstances would he be endorsing either of the two main parties' candidates (John McCain – Republican Party, or Barack Obama – Democratic Party).
What did Ron Paul refuse?
Ron Paul said that under no circumstances would he be endorsing either of the two main parties' candidates (John McCain – Republican Party, or Barack Obama – Democratic Party).

Question: What was his reasoning?
Self-contained rewrite: What was Ron Paul's reasoning for saying that under no circumstances would he be endorsing either of the two main parties' candidates in the 2008 presidential campaign?