Chart: Federal Tax Revenue to GDP Ratio
The United States government has never been able to collect taxes at a rate higher than 21%, and that was in 1944, the last full year of World War II.
Here is the chart.
This means that, politically speaking, Congress since 1944 has refused to impose taxes sufficient to extract over 20% of GDP. This is a ceiling of federal taxation in the United States. The only way that the federal government can increase spending above 20% of GDP is to borrow: from the public, from government agencies, from foreign central banks, and from the Federal Reserve System.