By Nicolás Misculin

BUENOS AIRES (Reuters) – Argentina’s conservative opposition is on track to win the Senate election race in the key swing province of La Pampa, three sources told Reuters on Sunday, which would be an important win to help erase the ruling party’s majority in the upper house.

The sources, from both parties, said that Juntos por el Cambio held a strong lead over the ruling Peronist party Frente de Todos in the province’s Senate race vote count, which they said was highly unlikely to change.

Argentines voted on Sunday in midterm elections that will establish the power balance in Congress, with the ruling Peronist party battling to avoid damaging losses that could erase its majority in the Senate held for almost 40 years.

Focus is on the lower-house result in densely populated Buenos Aires province, a Peronist stronghold where a defeat for Fernandez’s coalition would sting. There are key Senate races in provinces such as La Pampa, Chubut and Santa Fe.

(Reporting by Nicolas Misculin; Editing by Adam Jourdan)