They have said the percentages during the debates. The rich aren't going anywhere and even if they do they still have to pay taxes as an American citizen.
The rich have to pay their fair share because they make 20x the money of an average person but don't spend 20x the amount of money. They use loopholes to keep from paying their share then keep the rest to make more money instead of spending it and helping to stimulate the economy, this is why trickle down economics doesn't work.
"Fair share" often amounts to limiting/reducing tax loopholes and ensuring that capital gains -- which disproportionately benefit the well off -- are taxed at the normal rates of income. There may be some marginal tax rate at upper ends of the income bracket beyond that (all of which are well within historical norms or comparable rates to other OECD countries), but that's generally what the "fair share" euphemism amounts to.