United States Senate majority leader Mitch McConnell has set a test vote for stalled bipartisan legislation to let Congress review and possibly reject any agreement the Obama administration makes to ease sanctions on Iran in exchange for concessions on nuclear research and development.
The bill has been on the Senate floor, off and on, for more than two weeks.
It has been stalled since Republican senators Marco Rubio of Florida and Tom Cotton of Arkansas proposed politically attractive changes that drew the objections of Democrats as well as some Republicans who want the bill kept free of controversial provisions that could prompt the White House to withdraw its support.
A 60-vote majority on Thursday's test vote would likely discard both proposals, and greatly improve the bill's chances of passage.
Negotiators for the US and five other nations are rushing towards a June 30 deadline to finalise a deal in which Iran would curb its nuclear programme in exchange for relief from sanctions choking its economy.
Rubio, who is running for president, wants to amend the bill to require Iran's leaders to publicly accept Israel's right to exist, a nearly impossible mandate.
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As written, the legislation would block Obama from waiving congressional sanctions for at least 30 days while lawmakers weigh in on any final deal the US and five other nations can reach with Iran.
It also would stipulate that if senators disapprove the deal, Obama would lose the current authority he holds to waive certain economic penalties Congress has imposed on Iran.
Obama said he will sign it as written, but the White House warns that he will reconsider if the measure is substantially changed. More than 60 amendments have been proposed.
Negotiations seeking a definitive accord on Iran's nuclear programme will resume on May 12 in Vienna, the European Union and Tehran said on Tuesday.