Closing arguments are set to begin Monday afternoon in the corruption trial of Senator Robert Menendez, a powerful New Jersey Democrat accused of selling out his public office in pursuit of lucrative bribes.

The charges are among the most serious ever brought against a sitting U.S. senator. The government has accused Mr. Menendez and his wife, Nadine Menendez, of conspiring to trade his “influence and power” as Senate Foreign Relations Committee chairman to foreign governments and New Jersey businessmen in exchange for a Mercedes-Benz convertible, mortgage payments, gold and cash.

Mr. Menendez, 70, has pleaded not guilty to the charges, some of which carry penalties of up to 20 years in prison.

He is being tried in federal court in Manhattan alongside two of the businessmen, Fred Daibes and Wael Hana. Ms. Menendez’s trial was postponed after she was diagnosed with breast cancer.

The case has already dealt a near-lethal blow to Mr. Menendez’s four-decade political career. He did not run in the Democratic primary for his Senate seat last month. And while he has filed paperwork to run as an independent in November, polls show there is little chance he could win.