The U.S. Senate rejected an effort on Tuesday to require any nuclear agreement with Iran to be considered an international treaty, which would have forced any deal to be ratified by two-thirds of the Senate's 100 members.
The Senate voted 57-39 to reject the measure, which Republican Senator Ron Johnson offered as an amendment to the Iran Nuclear Review Act, a bill requiring an Iran nuclear deal to be reviewed by Congress.
The amendment's backing by 39 Republicans signaled that there could be intense debate in the coming days as the Senate hammers out its final version of the legislation.
Majority Leader Mitch McConnell and other top Senate Republicans were among those voting for the amendment, despite an emotional appeal against it from Senator Bob Corker, the Republican chairman of the Senate Foreign Relations Committee and author of the bill.
Corker and Senator Ben Cardin, the committee's top Democrat, have been working against so-called "poison pill" amendments seeking to toughen the bill, which they say would kill its chances of becoming law by alienating Democrats and provoking a veto by Democratic President Barack Obama.
Corker announced on Tuesday that his bill has 67 co-sponsors, enough to override a presidential veto.
Obama had threatened to veto the bill as a threat to ongoing nuclear negotiations with Iran until last week, when leaders of the foreign relations panel agreed on a compromise that removed many of the measure's strictest provisions.
Secretary of State John Kerry said Monday that Washington and other major powers were closer than ever to a deal with Iran, although more tough talks lay ahead of a June 30 deadline for reaching a final agreement in which Tehran would drastically scale back its nuclear program in exchange for an easing of crippling economic sanctions.
The White House has made clear the veto threat would be back in place if the measure were significantly amended as it moves through the Senate and House of Representatives.
Many Republicans worry that Obama is so eager for a nuclear pact that he will allow Iran to develop a nuclear weapon. They say a tougher stand in Congress would help convince Tehran to compromise in the nuclear talks.
Several backers of the bill insisted that supporting the Corker bill was not an endorsement of a final nuclear agreement. They argued that opposing it would take away Congress' best chance to weigh in.
