BUENOS AIRES: Sergio Massa, who placed third in the first round of Argentina's presidential election, will give a quiet nod of support this week to opposition candidate Mauricio Macri for the run-off vote, three Massa camp insiders said on Monday.

Macri delivered an unexpected hammer blow in Sunday's vote to the ruling Front for Victory party, whose candidate Daniel Scioli had been fancied to win outright but now faces a tight battle in the second round.

Massa won 21.3 percent of votes on Sunday. While it wasn't enough to get him through to the run-off, it puts him in an influential position over the next four weeks because both Scioli and Macri need to court his supporters.

Yet Massa is likely to shirk the role of king-maker because he cannot persuade his supporters to follow him en masse and trying to do so could driving a wedge through his party.

Instead, senior lieutenants in his New Alternative alliance are drawing up a manifesto detailing policy proposals that is due to be unveiled on Wednesday.

"The document won't explicitly support Macri, but it is a wink in his direction," said one close aide who is familiar with Massa's strategy. "The core tenets will be much closer to what Macri is proposing than Scioli."

One of the other sources said the team drawing up Massa's policy document would meet in the central city of Cordoba on Tuesday and Wednesday.

"The document will be easier for (Macri's) 'Let's Change' alliance to find common ground with than the ruling party." the source said.

The outcome of the Nov. 22 presidential run-off will shape how Argentina tackles its economic woes, including high inflation, a central bank running out of hard currency, an over-valued peso currency and a sovereign debt default.

Scioli promises "gradual change" to outgoing leader Cristina Fernandez's interventionist policies while Macri talks of swiftly dismantling trade and currency controls to convince investors to put their money in Latin America's No. 3 economy.

Scioli, a former powerboat champion, won 36.9 percent of votes on Sunday, while centre-right Macri took 34.3 percent.

Both sprinted out of the blocks on Monday to try to attract voters who had backed the four candidates eliminated in the first round.

Massa is playing his cards close to his chest. He served as Fernandez's cabinet chief during her first term but broke away and his party is a dissident faction of the Peronist movement that dominates Argentine politics. On the campaign, however, he also warned that Macri could drive the economy into a new crisis by imposing pro-market policies too abruptly.

"Argentines now have to decide a path to take in the next three weeks," Massa told his supporters at his campaign bunker late on Sunday. "We know what our role is and the responsibility we have."

Perhaps hinting where his preference lies, he capped off his speech by saying: "Tonight change begins."