The Trump administration rejected a proposal by Russian President Vladimir Putin for a one-year extension of a critical nuclear arms-control treaty Friday, dimming the chances of a diplomatic breakthrough before the Nov. 3 U.S. presidential election.

Putin offered to extend New START, a 10-year treaty that places limits on the two countries’ nuclear warheads, without preconditions at a meeting of his security council, but national security adviser Robert C. O’Brien called the proposal a “non-starter.”

“We hope that Russia will reevaluate its position before a costly arms race ensues,” O’Brien said in a statement.

The breakdown in negotiations comes as President Trump, trailing Democratic rival Joe Biden in national polling, urges his diplomats to bring him foreign policy victories.

The 2010 treaty, which expires in February, restricts the number of deployed strategic nuclear warheads and certain launch platforms. If the treaty isn’t extended or replaced, the world’s two biggest nuclear powers will return to an era without substantive restraints on their arsenals for the first time in decades.

As with other last-minute efforts to forge diplomatic breakthroughs before the election, such as a rapprochement between Sudan and Israel, U.S. diplomats appear in need of more time to work out the details.

On Friday, Putin said it would be “exceedingly sad” if the treaty expired. His foreign minister, Sergei Lavrov, blamed U.S. intransigence for demanding a large number of preconditions that go beyond the treaty.

The Trump administration didn’t start negotiations in earnest until earlier this year, prompting criticism from arms-control advocates who said discussions with the Russians should have begun much sooner.

The Trump administration’s arms-control envoy, Marshall Billingslea, initially insisted that China participate in talks. He wanted any replacement treaty to include China and to encompass all of Russia’s nuclear weapons — not just the “strategic” weapons covered under New START but also its sizable stockpile of smaller, “tactical” nuclear weapons that fall outside the treaty. Billingslea also insisted that verification mechanisms for any follow-on treaty be strengthened.

Russia rejected the demands, and China has refused to take part in negotiations.

Trump then dispatched O’Brien to meet with his Russian counterpart, Nikolai Patrushev, early this month in Geneva.

“We’ve come to a logjam in our meetings with the Russians on New START, and so we felt — the president thought it would be helpful if I went and spoke to my counterpart to break the logjam,” O’Brien said in an Oct. 5 interview with radio host Hugh Hewitt.

After that meeting and the calls between Trump and Putin, the Trump administration thought an agreement in principle had been reached, prompting Billignslea to divert a trip in Asia to Helsinki to again meet with his Russian counterpart. The administration was hoping to agree to extend New START for one or two years and in the meantime place a freeze on both countries’ full nuclear arsenals, Billingslea told reporters. But that deal hasn’t materialized.

Speaking Tuesday at the Heritage Foundation, Billingslea reiterated that he believed the United States and Russia had reached an “agreement in principle at the highest levels of our two governments” and said he hoped the “gentleman’s agreement” would “percolate down through their system so that my counterpart hopefully will be authorized to negotiate.”

“We’re ready to strike this deal. We could strike it tomorrow, in fact,” Billingslea said. “But Moscow is going to have to show the political will to do so as well.”

On Friday, Putin made no mention of a mutual freeze on the countries’ nuclear stockpiles, proposing instead a simple one-year extension of the treaty with no conditions while Moscow and Washington negotiate what comes next.

The White House rejected the proposal out of hand, saying it wanted a more ambitious agreement.

“The United States proposed an extension of New START for one year, in exchange for Russia and the United States capping all nuclear warheads during that period,” O’Brien said. “This would have been a win for both sides, and we believed the Russians were willing to accept this proposal when I met with my counterpart in Geneva.”

The treaty includes a clause that allows the leaders of both nations to extend the agreement by five years without requiring ratification. Both Putin and Biden have said they would agree to the five-year extension.

Despite the apparent breakdown in talks on Friday, Washington and Moscow could still revive the negotiations before the election. But the Russians have an incentive to wait and see the outcome of the vote, because Biden so far has signaled his willingness to extend the current treaty as it stands. If Biden wins, he would have about two weeks after inauguration to extend the pact.

Billingslea initially warned that the Russians would pay a price if they didn’t agree to a deal before the election and Trump emerged victorious, but he later appeared to back off that threat.

“We can have a deal tomorrow, or we can have a deal whenever. We’re ambivalent,” Billingslea said Tuesday. “The ball is in Russia’s court, but I do think this will be in Russia’s interest ultimately, so hopefully they’ll take the deal.”

On Friday, Billingslea signaled defeat in a tweet. “The United States made every effort,” he wrote. “It is disappointing that the Russian Federation backtracked on an agreement covering all nuclear warheads for the first time. This would have been an historic deal, good for the U.S., Russia, and the world.”

While it is unclear how much significance American voters place on U.S. foreign policy, Trump has sought to showcase his skills as a peacemaker in the final stretch of the election.

Last month, he tweeted that U.S. troops “should” come home from Afghanistan by Christmas, a timeline his military leaders have questioned. He also previewed that “five or six” nations were poised to make normalization agreements with Israel following the U.S.-brokered deals with the United Arab Emirates and Bahrain, but those have yet to materialize.