"Ed Huntress" wrote:

A very good question. And the answer is...nothing would stop them.

France tried it in 1971. That's one reason why we have fiat money now.

In response to the Great Depression, and at the request of President Franklin D.
Roosevelt, Congress passed the Gold Reserve Act on 30 January 1934; the measure
nationalized all gold by ordering the Federal Reserve banks to turn over their supply to
the U.S. Treasury. In return the banks received gold certificates to be used as reserves
against deposits and Federal Reserve notes. The act also authorized the president to
devalue the gold dollar so that it would have no more than 60 percent of its existing
weight. Under this authority the president, on 31 January 1934, fixed the value of the
gold dollar at 59.06 cents.

Bibliography

Friedman, Milton, and Anna Jacobson Schwartz. A Monetary History of the United States,
1867–1960. Princeton, N.J.: Princeton University Press, 1963.