WASHINGTON (AP) -- Democrats on Wednesday blocked the Senate's first spending bill of the year in a last-minute fight over a Republican effort to undercut the Iran nuclear deal and scuttle U.S. plans to buy Iranian "heavy water."


Still seething over last year's international pact, Sen. Tom Cotton, R-Ark., offered an amendment to the energy and water bill that would thwart the Obama administration if it tries to buy more heavy water from Iran. The administration completed an $8.6 million deal to buy 32 metric tons of heavy water from Iran last Friday.


The sale will help Iran meet the terms of last year's landmark deal in which Iran agreed to curb its atomic program in exchange for billions of dollars in sanctions relief. Heavy water, formed with a hydrogen isotope, is a key component for one kind of nuclear reactor and has a variety of research, medical and industrial applications.


Democrats called the amendment a "poison pill" that would draw a veto from President Barack Obama. A vote to move forward with the energy and water bill failed to reach 60 votes needed to continue, falling short 50-46.


White House spokesman Josh Earnest said the administration was gratified the amendment did not advance. Earnest took a swipe at Cotton, who last year antagonized the White House with a Senate GOP letter telling Iranian leaders that a nuclear deal with Obama could be undone by his successor.


"Senator Cotton is certainly no expert when it comes to heavy water. I'm confident that he couldn't differentiate heavy water from sparkling water," Earnest said. "His focus is on undermining the effective implementation of this agreement that prevents Iran from obtaining a nuclear weapon."


Cotton responded on Twitter, telling Earnest he was at least half right: "I don't know much about sparkling water. It isn't served in Army, unlike in your ritzy West Wing." Cotton served on active duty in Iraq and Afghanistan before being elected to Congress.


It was unclear Wednesday how the standoff will affect the overall appropriations process as Congress struggles to complete the 12 spending bills without divisive fights over policy.


Senate Majority Leader Mitch McConnell, R-Ky., has repeatedly made an orderly appropriations process his No. 1 goal for 2016, a key element in his election-year effort to show voters that Republicans can govern. That effort may now be in jeopardy, while over in the House, new GOP Speaker Paul Ryan has failed to accomplish the even more elementary task of passing a budget to serve as an appropriations blueprint, due to divisions in his party.


Sen. Dianne Feinstein, D-Calif., said lawmakers were blindsided by Cotton's amendment, adding that the proposal breaks a bipartisan agreement not to include controversial riders in the spending bill.


"Why can't he wait?" to put the amendment on a separate bill that is not part of the appropriations process, asked Feinstein.


Sen. Lamar Alexander, R-Tenn., chairman of an energy and water subcommittee, said Cotton was "entirely within his rights" to offer the amendment. Alexander and other lawmakers faulted the administration for failing to explain the heavy water purchase or give lawmakers a heads-up before it went through.


"The U.S. Department of Energy, without any consultation with anybody in the Senate that I know about, without the Intelligence, Armed Services, Foreign Relations committees, decided it was going to buy heavy water from Iran," Alexander said.


There are no current plans for further U.S. purchases of heavy water, and Cotton said his amendment would simply "hold this administration to its promise by ensuring that taxpayer dollars cannot be used again for the same purpose. We've given the terror-sponsoring Iranian regime enough concessions at the risk of our security. We should not further subsidize its enrichment activity by making repeated purchases of this material."


A sales agreement signed Friday in Vienna calls for the Energy Department's Isotope Program to purchase the heavy water for $8.6 million from a subsidiary of the Atomic Energy Organization of Iran. The heavy water will be stored at the Oak Ridge National Laboratory in Tennessee and then resold on the commercial market for research purposes.


Heavy water is not radioactive but has research and medical applications and can also be used to produce weapons-grade plutonium. Under the nuclear 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.


Sen. Chris Coons, D-Del., said a "fuller, earlier briefing" by the administration would have helped address lawmakers' concerns.


By AP