Menendez said Maduro, who took power after former Venezuelan President Hugo Chavez died of cancer last year, is governing through the use of fear and political retribution.
"The brutal and oppressive tactics President Maduro is subjecting his fellow countrymen to will only further diminish his legitimacy as president and the United States will not look the other way while the democratic aspirations of Venezuelans are viciously trampled," he said in a statement.
Opposition leader Leopoldo Lopez was charged by Maduro with terrorism, a charge Amnesty International said was politically motivated and without merit.
Venezuelan newspaper El Universal reported Thursday charges against Lopez were dropped after he surrendered to authorities earlier this week.
Maduro has faced mounting public and political criticism since taking office 11 months ago. El Universal reported the air in Caracas, the capital city, smelled of tear gas Thursday.
U.S. State Department spokeswoman Jen Psaki said Tuesday an order given by the Maduro government for U.S. consular officials to leave the country was an effort to draw attention away from its own problems.