Prosecutors had called their first witness, and Senator Robert Menendez’s blame-my-wife strategy in his federal bribery trial already appeared vulnerable.

An F.B.I. agent was testifying about a search of the New Jersey home that Mr. Menendez, 70, shares with his wife and the gold bars that were found inside a locked bedroom closet — bribes prosecutors say were paid to the couple in exchange for political favors by the senator.

Although the closet held women’s clothing, the agent said, a man’s blue blazer was also found hanging inside, seemingly tying Mr. Menendez to the gold and other alleged bribes. The agent stuck by his account even after Mr. Menendez’s lawyer, during cross-examination, pressed him and displayed a photograph showing the blazer hanging outside the closet.

But the next morning, the agent asked to clarify his testimony. After reviewing photographs of the search, he said, he agreed the blazer was hanging outside the closet.

Was his previous day’s testimony no longer accurate? Mr. Menendez’s lawyer asked.

“That is correct,” the agent said.

The moment passed quickly, but the exchange illustrated what has become a central pillar of Mr. Menendez’s defense: shifting blame to his 57-year-old spouse, Nadine Menendez, as he stands trial accused of trading political favors for hundreds of thousands of dollars in cash and gold.