"As you consider your vote, I want you to know that the President and US take this vote personally," Haley said. "The President will be watching this vote carefully and has requested I report back on those countries who voted against us. We will take note of each and every vote on this issue."
The unusual letter, with its veiled threat, isn't really about Jerusalem, analysts said, and certainly isn't about diplomacy. It is about domestic politics, as Haley navigates her way within the Trump administration and the administration itself throws red meat to the President's base.
Even so, Haley's rhetoric ruffled feathers and could have international consequences, they say, eroding her reputation at the UN, potentially undermining the US ability to build coalitions, and very likely dealing Trump an embarrassing blow on the global stage.
"Personalizing it, making it a vote for or against President Trump, is a bizarrely stupid tactic," said Richard Gowan, a New York based UN expert at the European Council on Foreign Relations. "I think there is still an extraordinarily high chance that ... the General Assembly will vote tomorrow by a very large margin to, in Haley's view, offend the President."
Trump himself made clear he will scrutinize the outcome. "We're watching those votes," the President said at the opening of a Cabinet meeting Wednesday. "Let them vote against us, we'll save a lot. We don't care."
"People that live here, our great citizens that love this country, they're tired of this country being taken advantage of and we're not going to be taken advantage of any longer," Trump added.
That declaration, said Khaled Elgindy, a fellow at the Brookings Institution, is about "playing to a domestic audience, which is what they were doing when Trump recognized Jerusalem as Israel's capital."
Trump has hit record low approval ratings amid the ongoing Russia probe and a tax overhaul opposed by 55% of Americans
according to a new CNN poll.
The rhetoric, Ghindy said, also was "not a message that foreign ministers are going to receive as some sort of attempt at diplomacy."
Indeed, reactions ranged from the publicly restrained to the privately exasperated. "This kind of threatening tone shows inexperience," one senior foreign diplomat said.
Heading for a 'super defeat'
Miroslav Lajčák, the Slovak president of the General Assembly, said that it's "the right and the responsibility of member states to express their views" and so, at the request of member states, the vote would go ahead.
The outcome isn't a mystery. "Virtually the entirety of the General Assembly is going to vote against the US position," said Gowan. "It's just heading for a super defeat."
Even US friends say they'll go their own way. "No matter how close we can be to the US, we are keen to keep up our longstanding attachment to UN Security Council resolutions on the Jerusalem status," said a diplomatic source. "It's really a question of abiding to international law, and this is where we disagree with the US."
Ghindy said the disagreement could ripple in ways that affect the Trump administration later. It could undermine its ability to get the help of other countries to pursue any policies on Iran or North Korea, he said. "Especially those that are a break from the past, like undoing the Iran deal or inching toward military action on North Korea," he said. "You're going to need partners."
Indeed, the senior foreign diplomat, who spoke anonymously to protect relations with the Trump administration, said that "nine times out of 10, it's the US asking the rest of the UN for support, not vice versa."
"For many members, especially Western ones, their votes reflect positions they've held for 50 years," the diplomat said. "The two-state solution, the status of Jerusalem through negotiation, etc. — they are supposed to abandon 50 years of policy, for what exactly?"
Throughout the first year of the Trump administration, UN diplomats have largely distinguished between Haley's rhetoric and her diplomacy in the UN system. She has been tart tongued and sharp elbowed when it comes to defending Israel, making clear that there's a new sheriff in town.
On her first day at the UN, Haley issued a warning -- to US allies: "For those who don't have our backs," she said, "we're taking names, and we will make points to respond to that accordingly."
Public versus private
As they got to know Haley, many foreign diplomats started to discount some of her rhetoric. They could see she needed to keep her domestic base happy, they said, and has political ambitions that extend beyond the UN.
Her public barbs were countered by a pragmatic and largely collegial approach in private. She has avoided the massive UN budget cuts Trump had threatened after a UN vote condemning Israeli settlements, and she demonstrated a deft diplomatic touch in getting China to sign onto North Korea resolutions.
"Most of the time, the diplomatic community in New York is willing to put up with her rhetorical flourishes," said Gowan. "This week is a bit different."
"She's standing up for a US policy that most people think is a massively unforced error," Gowan said. "I think it will probably do a little bit of damage to her ties to diplomats."
Some polling counters Haley's claim about the will of the US people. A University of Maryland Critical Issues poll released December 1 found that 63% of Americans oppose moving the embassy to Jerusalem, including 44% of Republicans. The pollsters questioned 2,000 people and had a margin of error of 2.19%.
A December 13 Monmouth University poll that found Trump at record low approval ratings also found that 39% of Americans thought moving the embassy to Jerusalem was a bad idea, while only 23% thought it was a good idea. Some 51% of respondents said it would lead to greater regional instability, while 10% said it would make the region more stable.
Haley's most recent round of tough talk might be driven by her relationship with Trump. "There's always been the sense that Haley is not a 100% paid up member of Team Trump," Gowan said.
The former South Carolina governor is more of a mainstream Republican internationalist than the President and has often taken a tougher line on human rights than the rest of the administration. In December, she raised eyebrows inside the administration when she said that women who accused Trump of sexual harassment deserve a hearing.
"I wonder if, having had some friction with the White House, she's not trying once again to prove her loyalty to Trump by really hammering the table on his behalf in New York," Gowan said.
"She's always walking this very, very fine line between staying close enough to the President to keep her job, but also far enough away from him that should he trip up and implode, her political career will not be too negatively affected," he said. "The personal politics matter a lot."