U.S. Senator Ben Cardin has said that he would vote against the nuclear agreement with Iran, lessening the chances that US President Barack Obama can win enough votes to avoid having to use his veto power to protect the international agreement.
The Maryland lawmaker became the third Senate Democrat to announce his opposition.
The others, Charles Schumer of New York and Robert Menendez of New Jersey, came out against it last month.
This leaves just five of Obama's fellow Democrats in the chamber undecided.