It had been a busy Thursday for Senator Robert Menendez of New Jersey.

He was in Washington presiding over the Senate Foreign Relations Committee as it heard testimony about the need for sustained aid to Ukraine, and then preparing to travel to Philadelphia with his wife, Nadine Menendez, to accept an award from an Armenian-American organization.

Back at home, the F.B.I. was watching.

An agent, conducting surveillance near the couple’s modest, split-level house in Englewood Cliffs, N.J., snapped a photo of a Mercedes-Benz convertible parked out front, a court filing shows. Several weeks later, investigators searching the home would find 13 bars of gold bullion and more than $480,000 in cash, much of it stashed in coat jackets, boots and a safe.

On Monday, almost two years to the day after that agent was watching the senator’s house, Mr. Menendez, a Democrat, is to go on trial in Federal District Court in Manhattan, charged with taking part in an elaborate, yearslong bribery scheme.

It will be his second corruption trial in seven years, but unlike the first, which ended in a hung jury, there is a volatile and surprising new element: charges against Mr. Menendez’s wife.

The case, prosecutors have indicated, is as much about Ms. Menendez as it is about her husband. The U.S. attorney’s office for the Southern District of New York has depicted Mr. Menendez and his wife as collaborators who took bribes in exchange for the senator’s willingness to steer weapons and government aid to Egypt, prop up a friend’s halal meat monopoly and meddle in criminal investigations involving allies.

“What else can the love of my life do for you?” Ms. Menendez asked at a Washington steakhouse dinner during one of the many meetings that prosecutors say she arranged between her husband and Egyptian officials.