Taxing Capital

With the election season beginning, I think there is a question that needs to be clearly answered:

Is there any legitimate reason why the tax schedules for capital (investment return) should be any different than the tax schedules for labor (wages)?

When the marginal tax rate for labor rises above 15%, shouldn't the marginal rate for capital income do the same? And shouldn't the total tax rate for an individual begin with the simple sum of labor and capital income, without distinction between them?

If one individual were to earn $75,000 from labor, and another $75,000 from investment return, is there any good argument that they should not pay the same taxes? Ditto if the amounts were $100k, $200k, etc?

Playing with the tax codes appears to be the most prominent campaign rhetoric of our candidates, a favorite tactic to gain acceptance from voters. So, it's only fair if we, the voters, really start having a conversation and determine beforehand what a good, fair tax code would look like, then we lessen the chance that we'll be hoodwinked into voting for a design that is unfair, and worse, damages our society.