The reasons that there should be a cap on how high the income tax rate can be at 33% is due to the Laffer curve and the history of taxes and their revenues. Many economists, on both the left and the right, agree that the apex of the Laffer curve is around 33%. This has been shown throughout history as when income tax rates are lowered, tax revenues have increased. This has been seen throughout the last four administrations that have cut the income tax rate. Under Coolidge, Kennedy, Reagan, and Bush Jr. tax rates were decreased which lead to an increase in tax revenue.
A Congressional Act would be written and passed putting a cap for federal income tax rates at 33%.