The two have both pledged to undo tight capital controls, cut spending, and allow the peso currency to fall, but diverge over how quickly to do so, with Larreta favoring a slower pace of transformation and Bullrich backing more shock tactics to right an economy where inflation is near 116% and dollar reserves almost dry.

In opinion polls the pair have been neck-and-neck to win the candidacy of the main Together for Change conservative coalition, with many voters still undecided.

"It's a very competitive election," said Facundo Nejamkis, director of pollster Opina Argentina, which predicts each candidate pulling in some 14-16% of the total vote on Sunday.

"We have it in a situation of parity, a very competitive primary... We will see what happens in the final days."

Argentine primaries are open, and seen as a dry run for the election. Recent polls have shown Sergio Massa, representing the ruling Peronist party, as the most popular individual candidate.

But the vote for Larreta and Bullrich combined is higher, so whoever wins out between the two in the Aug. 13 primary will be in pole position to claim victory in the Oct. 22 general election, or in a likely run-off between the top two candidates a month later in November.

Larreta is seen by analysts as more likely to lure moderate voters, while Bullrich could eventually appeal more to Argentines whose first choice is far-right libertarian Javier Milei, an outside candidate who is polling near 20% of the vote.

The rising support for the opposition comes amidst a dire economic backdrop. The soaring inflation has sapped wages, poverty is around 40%, and sky-high interest rates have dampened growth.

But tensions between the two conservative front-runners have helped Massa narrow the gap on them, and the two have tried to downplay any disagreements as the primary election approaches.

"We see today some of their vote is leaking away," said Mariel Fornoni, director of pollster Management & Fit.

'WE WILL ALL WORK TOGETHER'

Both agree on cutting government spending to cut a deep fiscal deficit and removing controls known as the "cepo" on the peso currency to allow it to float freely. Bullrich though backs a more abrupt shock for the economy with rapid measures.

"We are going to go much faster, more forceful and with measures from day one," Bullrich, 67, said in a recent presentation.

"(Bullrich) has a very clear vision of what needs to be done, which is to order the accounts," said Javier Iguacel, an adviser to the Bullrich campaign and a former energy minister.

Bullrich also proposes harsher penalties for criminals and promises to crack down on illegal road blockades during protests.

Larreta, 57, meanwhile, has pledged to go through the budget "line by line, peso by peso" to trim unnecessary spending and balance the books in his first year in office. He also wants to aggressively promote exports to bring in dollars.

Waldo Wolff, a former congressman and current Secretary of Public Affairs for Buenos Aires city under Larreta, said that the conservative mayor would look to long-term sustainable change.

"The core of Horacio's project has to do with making big agreements to sustain big transformations over time," he said.

Larreta, backed by the long-running and still powerful Radical Civic Union (UCR) and the Civic Coalition, is also targeting an overhaul of tax and labor laws.

"What we're discussing are ideas, how we are going to carry out our government plan. That is what this primary is for, but on the morning of the 14th we will all be working together, whoever wins," Larreta said in written comments to Reuters.

"We are all going to work together to win the general elections and begin to transform Argentina forever."

Iguacel, the adviser to Bullrich, struck the same tone.

"The one who wins leads and the one who loses will accompany them. This is an internal (primary), nothing more," he said.