The corruption trial of Senator Robert Menendez, a powerful New Jersey Democrat, spun into motion in Manhattan on Wednesday, with combative opening statements and an extraordinary claim by the defense.

Speaking directly to the jury, a U.S. prosecutor asserted that Mr. Menendez “put his power up for sale,” trading favors involving Egypt and New Jersey businessmen for gold bars, cash and a Mercedes-Benz convertible. But it was a lawyer for Mr. Menendez who shook the courtroom awake, piling blame on the senator’s wife, Nadine Menendez.

Mr. Menendez, 70, betrayed little emotion as he watched the opening statements from the courtroom, where he is facing some of the gravest charges ever leveled against a sitting U.S. senator. He has pleaded not guilty.

He is being tried alongside two of the businessmen, Fred Daibes and Wael Hana. Prosecutors have also charged Ms. Menendez, but her trial was delayed until July for health reasons.

Here are five takeaways from the senator’s third day on trial:

The prosecution tried to keep it simple.

Prosecutors have spun a dizzying set of accusations against Mr. Menendez, filing four rounds of charges that involve a halal meat monopoly, a Qatari sheikh and the inner workings of the U.S. government. All of it could easily confuse jurors.

So laying out a road map for their case, they offered the panel a far simpler view: “This case is about a public official who put greed first,” said Lara Pomerantz, an assistant U.S. attorney. “A public official who put his own interests above the duty of the people, who put his power up for sale.”