A year-and-a-half after President Barack Obama breathed new life into global talks on a climate treaty, the US is back in a familiar role - the holdout.
The Senate's decision last Thursday to shelve legislation on climate change is certain to cast a long shadow over December's meeting in Cancun, Mexico that will work on a successor to the Kyoto Protocol.
Obama's Democratic allies acknowledged they lacked votes to approve the first-ever US plan restricting carbon emissions blamed for global warming. The task is unlikely to get easier soon, with Democrats facing tight congressional elections in November.
"This is going to change the mood dramatically in terms of what countries are willing to put on the table in Cancun," said Jake Schmidt, international climate policy director at the Natural Resources Defense Council, which backs action to curb global warming.
"This will seriously downplay what we can realistically achieve."
Obama vowed to act on climate change when he was elected president, sharply reversing course from his predecessor George W. Bush, who was a sworn foe of the Kyoto Protocol, which he considered biased against wealthier countries.
Obama's climate negotiators enjoyed rousing welcomes when they arrived on the scene - especially from the EU, Kyoto's most enthusiastic champion.
The State Department, which leads international negotiations, said the Obama administration still considered climate a "priority" and would engage with other countries and with Congress.
"This is a global challenge and we have to resolve it through global cooperation and joint action by all of the key countries and key emitters. We are one of them," agency spokesman Philip Crowley said.
"And central to our ability to do our part is passing climate and energy legislation."
The clock is ticking on sealing a new treaty, with the Kyoto Protocol's obligations for rich nations to cut emissions expiring at the end of 2012.
Climate talks, including the contentious Copenhagen summit in December, have been plagued by fighting between wealthy and developing nations, which are both looking for clear commitments from the other side.
Major emerging nations have resisted any legally binding requirements to cut emissions and pressed first for industrialized powers to seal their commitments.
"Countries like China and India are not likely to commit to any sort of binding obligation if the US is not part of the discussion, part of the negotiation and makes some similar commitment," said Daniel Fiorino, an expert on environmental politics at American University.