MEXICO CITY (Reuters) - Claudia Sheinbaum's landslide win gives her an unprecedented mandate in Mexico, but success in handling the country's key ally and trading partner the United States hinges on how she can navigate the relationship and a possible clash if Donald Trump returns to the White House, according to former diplomats and analysts.

A climate scientist who will become Mexico's first female president, Sheinbaum is closely aligned with Mexico's outgoing President Andres Manuel Lopez Obrador. He has not always seen eye to eye with Washington but found surprising common ground with Trump in his first term despite tensions on trade and migration.

The smooth ties between the two populists is unlikely to be replicated with Sheinbaum, analysts and ex-diplomats said, given her more reserved and disciplined style.

"I don't think she will have the type of bromance that Lopez Obrador had with Donald Trump," said Arturo Sarukhan, who was Mexico's ambassador to the United States from 2007 to 2013.

That could make things more difficult if the U.S. Republican candidate defeats President Joe Biden in the November election. Polls show Trump leading in some battleground states.

But Sheinbaum will be all too aware that her promises to expand welfare programs and continue the policies that have helped her party lift millions of Mexicans out of poverty depend on having a stable and growing economy; and that requires the United States.

In 2023, Mexico surpassed China to become the United States' largest trading partner, and some 80% of Mexico exports are to the United States.

"The economy of Mexico largely depends on the United States," said Mexican political analyst Viri Rios.

"So part of her mandate is to keep a solid trade relationship because the best jobs in Mexico come from the export-oriented economy."

TRADE RENEGOTIATION

A review of the U.S.-Mexico-Canada Agreement (USMCA) is set for 2026 and will be a massive test to the relationship under Sheinbaum especially under a protectionist Trump administration, even though the pact itself is the product of a renegotiation that he orchestrated and boasted of during his first term.

Kenneth Smith Ramos, Mexico's former chief negotiator for the USMCA, said there will be a number of sticking points, including energy policy and a small but growing Chinese investment in strategic sectors in Mexico.

"I can perfectly imagine a second Trump presidency where President Trump comes to Mexico and says, OK, you have to decide, do you side with the U.S. or are you with China?" he said.

In addition to trade politics, Washington pressures the Mexican government to crack down on drug production and trafficking - especially of fentanyl - and on the movement of migrants north to the U.S. border. Mexico, in return, wants the United States to stop guns from being smuggled south into Mexico.

"The interesting thing is how all these issues work together," said Christopher Landau, who served as U.S. ambassador to Mexico during the Trump presidency.

"Assuming Trump wins, I think a day one issue is going to be the border and migration. So if Mexico is cooperative on that, that's certainly going to earn a lot of goodwill on the trade side," he said.

But it is not just under Trump that migration flows have become a bilateral bargaining chip.

Biden, for example, is expected to sign an executive order on Tuesday that could allow authorities to stop receiving asylum requests and deny entrance to migrants once a daily threshold is exceeded, a policy that would result in migrants from around the world who cross the southern U.S. border being returned to Mexico.

The announcement of such an executive order only days after Mexico's election shows how Mexico has been strong-armed by both Democratic and Republican U.S. administrations into enforcing U.S. policy south of the border, advocates and analysts say.

Still, the pressure on Mexico to ramp up detentions and deportations of migrants would likely increase under a second Trump term.

Trump and other Republicans have also escalated their rhetoric about mobilizing troops or firing missiles to combat Mexico's drug cartels - moves the Mexican government would vehemently oppose as a matter of national sovereignty.

And beyond personality styles and entrenched politics, there's the more simple fact that Sheinbaum will be Mexico's first woman president - a widely celebrated achievement that might nevertheless complicate her relationship with Trump.

"Claudia Sheinbaum is a scientist, a political leader and she self-identifies as a feminist," said Gema Kloppe-Santamaria, Assistant Professor of Latin American History at George Washington University.

"All of these qualifications make her a potential candidate to irritate Trump, if he is elected."

(Reporting by Laura Gottesdiener and Stephen Eisenhammer; Editing by Christian Plumb and Alistair Bell)

By Laura Gottesdiener and Stephen Eisenhammer