There isn't someone reading minds who realizes the country has a majority of people who would prefer slavery be illegal

The politicians have incentive to know what the majority of people think, and they are quite capable of asking people.

and calls a vote to happen on the issue of "should slavery be a thing".

If a candidate makes it their platform that they'll free the slaves, that's basically what the vote will come down to. If it's clear enough, both candidates will have it in their platform, and there won't even be a vote.

you would have to campaign and convince people both that they would be better off without slavery,

They seemed to work out that they don't want immigrants, women, and children taking their jobs. It didn't happen until the industrial revolution, but they weren't taking their jobs until then. Why would slaves be different?

AND that this means it should be illegal.

I know they made it illegal for children to work. I don't remember hearing anything like that about women, though. Also, once slavery started getting outlawed in a few states, you'd think people would be more willing to outlaw in it other states.

There are reasons why people don't form coalitions of 80 percent to rob the 20 percent.

I remember something about one state mandating that all contracts could be payed in cash (as opposed to gold), knowing full well that their cash was worthless. They essentially forced all the banks to forgive the debts of the farmers.