What exactly happened to tax revenues during the Clinton years? Megan McArdle offers a primer:

First of all, while it is technically true that the federal tax take was “around 20% of GDP” during the Clinton era, this was only true at the height of the stock market bubble. Tax revenues exceeded 20% of GDP for exactly one year: 2000. The average tax take under Clinton was 19%. And if you exclude 1999 and 2000, the very height of the bubble, it was more like 18.5%.
Without arguing about whether our tax system is fair or not, the fact is that the federal income tax is the most variable part of the code; and the federal income tax is now very progressive; it collects most of its revenue from people at the top. (Whether it should collect even more is an argument for another day.) Because it collects most of its income from people at the top, and because the incomes of the wealthy are more variable than the incomes of the poor and middle class (Warren Buffett’s income can drop by $300,000; mine can’t), we’re going to get deep troughs in recessions, and high peaks in boom times. We will get particularly high peaks when the booms are delivering huge chunks of income to a handful of people in a very short timeframe. According to the CBO, capital gains receipts alone, which more than doubled in Clinton’s second term, accounted for more than 30% of the increase in income tax receipts above the rate of GDP growth. Obviously the ancillary ordinary income, like banking fees, also contributed substantially. Between 1996 and 2000, payroll taxes increased a tidy 30%. But income taxes increased by 55%. In 1996, social insurance receipts were about $500 billion, while income tax receipts were $650 billion. By 2000, payroll tax receipts had grown to $656 billion–but the income tax was collecting over a trillion. Today they’re roughly at par again (though that won’t last–social insurance contributions will drop as the worker to population ratio declines.)
Saying “all we have to do is go back to the tax rates under Clinton” is effectively saying “all we need is another asset price bubble that funnels a huge amount of money into the pockets of the rich”. This seems neither particularly feasible, nor desirable.

Nor does it seem like a scenario we’d want to bank on.