Why is there both a judge and a ballot box in your analogy? In the primaries, there were only ballot boxes, and Clinton got almost 30% more ballots from normal people outside of the DNC than Sanders. It was a landslide so large that the media had to construct a horse race story just to try to keep it interesting.

The DNC had "groomed" Clinton insofar as she had run in the primaries before and was familiar with the rules. She didn't receive any special treatment from the DNC during the primaries as far as their emails showed.

Because the judges put their votes in a ballot box. Or because stuffing a ballot box is an idiomatic phrase.

Rigging implies fraud, deception, or dishonesty, not just unbalanced.

https://www.merriam-webster.com/dictionary/rigged

See sense 3

1. to manipulate or control usually by deceptive or dishonest means <rig an election>

2. to fix in advance for a desired result <rig the contest>

Editted to add second part of sense 3.

Why did you skip the second part of sense 3, which is "to fix in advance for a desired result"?

I don't see that it adds anything to contradict the idea that rigging is only something "unbalanced" and not connoting deceit. "Fix in advance" is an example of fraud. I have no issue with including it in my comment, and have updated it to do so.

I'm not sure why you would think otherwise, though if you're implying that the example of extra, specialized training you provided above is an example of "fix in advance", I disagree. To "fix in advance" would mean that the outcome is predetermined, such as by bribes. This sense of fix is here:

https://www.merriam-webster.com/dictionary/fix

7b. to influence the actions, outcome, or effect of by improper or illegal methods <the race had been fixed>

If you have some other meaning implied by your comment, please explain. I find I'm a terrible mind reader.