Cristina Fernandez de Kirchner - a former president who has an ardent support base but is unloved by global investors - surprised locals and international observers alike on Saturday by saying she would run as vice president alongside former cabinet chief, Alberto Fernandez, a veteran political operator who has both backed and criticized her in the past.

Economists said the shock move could add to market uncertainty in the short-term but it should overall help temper volatility ahead of national elections in October.

The former president would likely still wield much power given the lower profile of her running mate, but being forced to put a more moderate ally above her on the ticket should create a buffer for any more contentious policies.

Cristina Fernandez is considered by investors to be a riskier prospect because of her past populist policies. She introduced currency controls and tax increases on farm exports while in office between 2007, and 2015.

"Taking a CFK (Cristina Fernandez) presidency off the table is likely to be welcomed by investors," Capital Economists said in a note on Monday, adding it "reduced the odds of a disorderly post-election default," a risk that had spooked investors.

It could though strengthen the challenge against centre-right incumbent Mauricio Macri if the Fernandez-Fernandez running pair can win over the more moderate wing of the broad Peronist opposition.

"To the extent that the move helps to unite the Peronists, it reduces the more investor-friendly President Macri's chances of re-election," Capital Economics added.