Donald Trump will announce his decision on whether to pull the United States out of the Paris climate accord during a White House Rose Garden event today.
The US president promoted his announcement on Twitter after a day in which American allies around the world sounded alarms about the likely consequences of a US withdrawal.
The announcement is timed for 8pm Irish time.
Mr Trump himself kept everyone in suspense, saying he was still listening to "a lot of people both ways".
The White House signalled that Mr Trump was likely to decide on exiting the global pact - fulfilling one of his principal campaign pledges - though top aides were divided.
The final decision may not be entirely clear-cut: aides were still deliberating on "caveats in the language", one official said.
Everyone cautioned that no decision was final until Mr Trump announced it.
The president has been known to change his thinking on major decisions and tends to seek counsel from both inside and outside advisers, many with differing agendas, until the last minute.
Abandoning the pact would isolate the US from a raft of international allies who spent years negotiating the 2015 agreement to fight global warming and pollution by reducing carbon emissions in nearly 200 nations.
While travelling abroad last week, Mr Trump was repeatedly pressed to stay in the deal by European leaders and the Vatican.
Withdrawing would leave the United States aligned only with Russia among the world's industrialised economies.
American corporate leaders including Apple, Google and Walmart have also appealed to the businessman-turned-president to stay and even fossil fuel companies such as Exxon Mobil, BP and Shell say the United States should abide by the deal.
Mr Trump's predecessor Barack Obama enacted the deal without Senate ratification.
A formal withdrawal would take years, experts say, a situation that led European Commission president Jean-Claude Juncker to speak dismissively of Mr Trump on Wednesday.
Mr Trump does not "comprehensively understand" the terms of the accord, though European leaders tried to explain the process for withdrawing to him "in clear, simple sentences" during summit meetings last week, Mr Juncker said in Berlin.
"It looks like that attempt failed. This notion, 'I am Trump, I am American, America first and I am getting out', that is not going to happen."
Some of Mr Trump's aides have been searching for a middle ground, perhaps by renegotiating the terms of the agreement, in an effort to thread the needle between his base of supporters who oppose the deal and those warning that a US exit would land a blow to the fight against global warming as well as to worldwide US leadership.
Secretary of state Rex Tillerson has favoured remaining in the agreement and chief strategist Steve Bannon supports an exit, as does Environmental Protection Agency administrator Scott Pruitt.
Mr Trump's chief economic adviser Gary Cohn has discussed the possibility of changing the US carbon reduction targets instead of pulling out of the deal completely.
Senior adviser Jared Kushner, Mr Trump's son-in-law, generally thinks the deal is bad but would still like to see if emissions targets can be changed.
Mr Trump's influential daughter Ivanka's preference is to stay, but she has made it a priority to establish a review process so her father would hear from all sides, a senior administration official said.
The emissions goals are voluntary, with no real consequences for countries that fail to meet them.
That means the US could stay in the accord and choose not to hit its goals or stay in the pact but adjust its targets for reducing greenhouse gas emissions.
America has agreed to reduce its emissions by 2025 to 26-28% of 2005 levels - about 1.6 billion tons.