Meaning – What Does “I Have Straight A’s” Mean?

I wouldn't vote for Clinton if you paid me

The use of would and paid are examples of backshifts, which indicate that this is a hypothetical condition, not a real one. Furthermore, not if you paid me is a well-known idiom meaning under no circumstances.

The bit that makes the meaning clear is an implied even

I wouldn't vote for Clinton even if you paid me.

What it now means is that under any and all circumstances, even if you were to offer me money to do it, I would not vote for Clinton.

I believe this is a vestige of hunting lingo, to ride straight to hounds, hunting being a typical sport of the country squire. It means to ride to the hounds (in a fox hunt) without taking a circuitous path in order to avoid obstacles such as ditches and hedges; riding straight to the action is emblematic of a robust forthrightness of character. It is analogous to being a straight shooter in American English.

Compare this excerpt from an 1865 story:

"What a good fellow Galton is", he began; "he has all the qualities and all the qualifications that both men and women like."

"He can ride straight to hounds, and hit a bird if he aims at it", she replied laughing.

"Women—and men too—like a fellow who can ride and tell the truth without swerving", he answered.

"There is an impression abroad that we weak-minded women 'go in', as you call it in your slang, for the athletic," she rejoined.