US President Donald Trump warned on Tuesday he could sever struggling Argentina's financial lifeline if his libertarian ally Javier Milei loses crucial legislative elections later this month.

"If he loses, we are not going to be generous with Argentina," Trump said as firebrand Milei visited the White House to seek the Republican's political and economic support.

"I'm with this man because his philosophy is correct. And he may win and he may not win – I think he's going to win. And if he wins we are staying with him, and if he doesn't win we are gone."

Trump's administration has already promised US$20 billion to prop up Argentina's struggling economy but his backing has failed to calm the markets – or help Milei's polling ahead of midterms on October 26.

The results of the elections, in which Milei's minority party is hoping to boost its seat tally, will dictate whether Milei can pass tough cost-cutting reforms or will face a legislative brick wall for the next two years of his term.

Hailing Milei as a "great leader," Trump said he would "fully endorse" his ideological ally in the elections.

"He's Maga all the way, it's 'Make Argentina Great Again,'" Trump said.

Trump has however faced questions about how a big bailout for Argentina tallies with that same America First policy.

Asked by reporters what the benefit to the United States was, Trump replied: "We are helping a great philosophy take over a great country. We want to see it succeed."

With Argentina struggling to stave off yet another financial crisis and Milei's disapproval ratings rising, the chainsaw-wielding leader has come to right-wing ally Trump for help.

Trump has repeatedly voiced political support for Milei, while backing it up with a promise of huge economic aid, but the markets remain spooked by Argentina.

In recent weeks, highly indebted Argentina has had to spend more than a billion US dollars to defend the peso, a strategy most economists believe is unsustainable.

That prompted Milei's allies in Washington to step in with a financial bailout.

"Argentina faces a moment of acute illiquidity," US Treasury Scott Bessent said last week, announcing a deal that would give Argentina access to US$20 billion.

"The US Treasury is prepared, immediately, to take whatever exceptional measures are warranted to provide stability to markets."

The announcement sparked a rally in Argentine bonds and stocks and helped ease pressure on the peso.

It also marked a rare instance of direct US intervention in Latin American currency markets, underscoring Washington's strategic interest in Milei's success.

"The United States saw this attack on Argentina, on the ideas of freedom, on a strategic ally – and that's why they supported us," Milei said in a radio interview on Monday.

"They know we are a true ally," Milei said, referring to Argentina's alignment with US and Israeli interests. (AFP)