The proposal fell by a vote of 47-53, reported AP.
All Republicans, who are in the majority, voted no, except Sen. Rand Paul, who joined Democrats in voting yes. Sen. John Fetterman, a Democrat, in turn voted no along with the Republicans, according to the BBC.
The Trump administration has chosen to carry out attacks on Iran without congressional approval. A declaration of war must be approved by Congress.
A vote is scheduled in the House of Representatives on Thursday.
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