In an essay published by the state-run newspaper Granma, Castro remarked that President Barack Obama suffered what he called "the biggest defeat in memory in the last three-quarters of a century" in the Nov. 2 vote.
He wrote that he considers Democrats' loss of control of the House of Representatives to be the result of racism and frustration among Americans.
"The most reactionary sectors in the United States are sharpening their teeth," Castro said, predicting that "all power (will fall) to the extreme right in the United States."
The 84-year-old leader frequently publishes his thoughts in newspaper columns since stepping aside in favor of his younger brother Raul in 2006. In his latest missive, he also criticized the Fed's announcement that it would buy $600 billion in Treasury bonds over the next eight months to try to energize the world's largest economy.
The plan raised alarm in capitals from Berlin to Beijing, with critics saying it will drive down the dollar's value and give U.S. goods an unfair competitive edge in world markets.
"Apparently the U.S. government, with its traditional anti-crisis measures, resorted to another desperate decision," Castro said.
Obama defended the Fed on Friday in Seoul following the G-20 summit, saying, "From everything I can see, this decision was not one designed to have an impact" on the dollar. "It was designed to grow the economy."
Castro also questioned the G20 summit itself, which gathered 32 leading economic countries to discuss world economic and financial issues.
For the rest of the world's nations, Castro lamented, "No forum exists where they can say a word about the imperial looting of their resources and about their urgent economic needs."