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BUENOS AIRES (Reuters) - Argentina’s fiery former President Cristina Fernandez formally launched her bid on Saturday for a Senate seat in October’s mid-term elections in a race that will determine whether President Mauricio Macri can deepen his free-market reforms.

Fernandez’ run, confirmed by a spokesman, comes a little over a week after she formed a new center-left party independent of Peronism, the Argentina’s dominant political movement that she has belonged to for decades.

While Fernandez enjoys strong support from many who consider her a champion of the poor, her break with Peronism divides the opposition and could help Macri’s coalition make a strong showing in October.

Fernandez will be competing for a Senate seat representing the populous province of Buenos Aires, home to 40 percent of the country’s voters and closely watched as a defining race.

Her political ally Daniel Scioli, a former governor of the province of Buenos Aires who lost the 2015 presidential election to Macri, will also run for a Senate seat in Buenos Aires, the spokesman said.

Fernandez implemented generous welfare programs, nationalized businesses and imposed currency controls as president of Latin America’s third largest economy between 2007 and 2015.

Last year Fernandez was indicted on charges she ran a corruption scheme with a public works secretary who was arrested a year ago while trying to stash millions of dollars in a convent. A position in Congress would give her immunity from arrest, though not from trial. Fernandez dismisses the accusations as politically motivated.

While October’s legislative elections are not expected to change the balance of power in Congress, where no party has a majority, they are seen as a barometer of current support for Macri, who could seek reelection in 2019.

Opinion polls have shown mixed results for the race in which a third of the Senate and almost half of the Chamber of Deputies will be elected.