The U.S. House of Representatives passed legislation on July 13 to undercut the Iran nuclear deal by blocking the purchase of "heavy water" from Iran as authorized under the agreement.
The 249-176 vote for the measure, with support coming almost exclusively from Republicans, who hold a majority in the chamber, defied a veto threat from President Barack Obama, who has vowed to defend the deal announced a year ago.
The House is due to vote on two other measures aimed at derailing the nuclear deal later this week.
Every congressional Republican and a few Democrats opposed the deal between Iran and world powers to limit Iran's nuclear activities to peaceful ones in exchange for relief from crippling international sanctions.
Heavy water is a nonradioactive byproduct from making nuclear weapons and nuclear energy. Under the deal, Iran is allowed to use heavy water in its modified Arak nuclear reactor but must sell any excess supply of both heavy water and enriched uranium on the international market.
Heavy water can be used in research and medical applications but can also be used to produce weapons-grade plutonium.
The Obama administration in April agreed to buy $8.6 million worth of heavy water from Iran to help it fulfill the terms of the agreement. The White House said removing Iran's surplus heavy water would prevent Tehran from ever using it to produce nuclear weapons.
The sale angered Republicans, however, who said Iran should be punished for continuing to manufacture heavy water, not rewarded with millions of U.S. taxpayer dollars that may end up being used to support terrorism or Tehran's ballistic missile program.
"Why are we giving the seal of approval to Iran's heavy water production?" asked Representative Ed Royce, chairman of the House Foreign Affairs Committee.
Royce denied that the House measure would violate the landmark pact. He said it would deter Iran from producing heavy water by making its sale more difficult.
Representative Eliot Engel, the committee's senior Democrat, also opposed the nuclear deal and said he agreed the United States shouldn't be buying Iran's heavy water.
But Engel and other Democrats accused Republican leaders of engaging in "political theater" by scheduling the votes on the Iran deal just before the Republican National Convention next week, at which billionaire Donald Trump is expected to win the party's presidential nomination.
Trump, like other Republican leaders, has vowed to tear up the Iran deal if he becomes president in January.
But as long as Obama is in the White House, the House bill "has no chance of becoming law," Engel said. Moreover, he noted that a measure similar to the House bill died in the Senate two months ago.
Republicans argued that Obama was so eager to bolster his foreign policy legacy that he agreed to sanctions relief Iran did not deserve because it lied about its past attempts to develop nuclear weapons, supports militant groups that attack U.S. allies, and perpetrates human rights abuses.
The White House and Democrats who supported the nuclear deal in Congress see it as the best way to defuse a long-running dispute over Iran's nuclear program that threatened to further destabilize the Middle East.
With reporting by AP and Reuters