NEW YORK (BLOOMBERG) - United Nations diplomats surprised by how they managed to cope with President Donald Trump's "America First" agenda are worried that things are only going to get tougher in 2019.

It's not State Department spokeswoman Heather Nauert - Mr Trump's pick to replace departing Ambassador Nikki Haley - who's raising concerns.

It's her boss, Secretary of State Michael Pompeo, and National Security Adviser John Bolton, both of whom are expected to have more sway over Ms Nauert than they did over Ms Haley, the former South Carolina governor who reported directly to the President.

Stoking those concerns are Mr Pompeo's recent comments questioning the value of the UN, Mr Bolton's scepticism of the organisation's work dating back to his days as the UN envoy for President George W. Bush, and Mr Trump's decision to downgrade the role of the next ambassador from Cabinet-level status.

"Unlike Haley, I don't think Nauert will be her own person," said Dr Stephen Stedman, an international relations professor at Stanford University who's worked for the UN.

"Other missions will correctly perceive her as having little independent weight."

US VERSUS UN

Mr Trump's first two years haven't been easy for the global body: the US has worked to lower the organisation's spending, pulled out of the UN-sanctioned Iran nuclear deal, cut off funding for an agency supporting Palestinian refugees, withdrawn from the Human Rights Council, and worked to undermine the Paris climate change accord.

Still, Ms Haley forged a strong relationship with Secretary-General Antonio Guterres, finding ways to dovetail Mr Trump's demands for UN spending cuts with Mr Guterres' efforts to trim ineffective and expensive peacekeeping programmes.

And at a time when Mr Trump was seen as equivocating on how hard to criticise Russia, Ms Haley was outspoken in attacking Moscow over its role in Syria, Ukraine, the poisoning of a former spy in the UK, and meddling in the 2016 US presidential election.

By stepping into a downgraded position, Ms Nauert may struggle to achieve similar success.

POMPEO'S DOUBTS

A Western European diplomat at the UN, who asked not to be identified criticising the US, said Ms Nauert's lack of diplomatic experience - before serving as State Department spokeswoman she was best known as an anchor at Fox News - means she'll depend more on instructions from Washington.

Ms Nauert declined to comment. A person close to Ms Nauert who asked not to be identified discussing her nomination, downplayed criticism of her experience and instead cited her close working relationship with Mr Pompeo, including joining him on three trips to Pyongyang as well as visits to countries such as Israel and Afghanistan.

Yet the UN envoy change comes as Pompeo signals a hardening view of the UN.

He said in a speech in Brussels on Dec 4 that the body "was founded as an organisation that welcomed peace-loving nations. I ask: Today, does it continue to serve its mission faithfully?"

"The UN's climate-related treaties are viewed by some nations as simply a vehicle to redistribute wealth," Mr Pompeo said in a speech entitled, Restoring the Role of the Nation-State in the Liberal International Order.

"Anti-Israel bias has been institutionalised. Regional powers collude to vote the likes of Cuba and Venezuela onto the Human Rights Council."

A huge issue looming for Ms Nauert, if she wins confirmation, will be sustaining tough international sanctions on North Korea. That'll require intense behind-the-scenes jockeying with rival ambassadors from Russia and China, instead of the front-of-the-camera type of work Ms Nauert has been accustomed to.

WHITE HOUSE TIES

Her supporters say Ms Nauert, 48, has shown herself to be a quick study. She leveraged her contacts with the White House - notably, Mr Trump's daughter Ivanka and son-in-law Jared Kushner - to survive the tumultuous early months of her tenure, when her first boss, then-Secretary of State Rex Tillerson, shunned her and she considered quitting.

That paid off when Mr Tillerson was fired in March and Ms Nauert was appointed acting undersecretary for public diplomacy and public affairs.

The designation made her one of the most senior officials in a pared-down department, and put her in charge of US outreach across the globe.

That included oversight of about 950 employees and a budget of US$1.2 billion (S$1.65 billion).

Mexican Ambassador to the UN Juan José Gómez Camacho said he isn't worried that Ms Nauert's appointment may reflect a change in America's stance toward the organisation.

"Nauert will be a great diplomat and a great communicator," Mr Gómez Camacho said in an interview. "Ambassadors represent countries and their specific policies, so I assume she will be representing the same policies advanced by Nikki Haley."

MYANMAR REFUGEES

While Ms Nauert hasn't spoken publicly about what her agenda will be, her priorities at the State Department suggest some possibilities.

In 2017, she visited Rohingya refugees who'd fled Myanmar to Bangladesh. She also championed the cause of the White Helmets, the volunteer rescue force in Syria, and frequently tangled with Russia on Twitter.

A person close to Ms Nauert, who asked not to be identified discussing her nomination, said she would carry on Ms Haley's efforts to "reform" the UN and defend Israel, as well as emphasise issues of human rights and religious freedom.

First, though, Ms Nauert has to get through the Senate confirmation process. Republican Senator Marco Rubio of Florida, a member of the Foreign Relations Committee, echoed a common concern among foreign policy commentators and policy makers when he questioned on CBS's Face the Nation whether she has the experience needed to do the job effectively.

But Ms Nauert's supporters are optimistic. They say she's learned a lot about foreign policy in her time at the State Department.

"She's terrific. I think she's a fast learner. Quick. Fresh eyes, which I think the UN desperately needs," said Mr Woody Johnson, the US ambassador to the UK and owner of the New York Jets NFL team.

Ms Nauert will be "somebody who can follow the great example of Nikki Haley and what she did", Mr Johnson said.

"She went in with fresh eyes, and I think she accomplished an incredible amount in a short period."