The death of Hector Olivares, 61, was announced by his ruling party coalition, called Cambiemos.
Six people have been detained, including two men arrested on Friday for the shooting which happened a day earlier and left Marcelo Yadon, a civil servant, dead at the scene.
He had been walking with Olivares.
Authorities say they do not think the shooting was politically motivated.
Yadon, 58, was hit five times at close range as he walked in a square in front of the National Congress with Olivares, a childhood friend.
Olivares was struck in the abdominal area and his condition had been touch-and-go ever since.
The motive of the shooting is still not known.
Press reports in Argentina have said it may have been linked to a relationship that Yadon was having with a much younger daughter of one of the assailants.
The shooting was caught on security cameras. Shots were fired at Olivares and Yadon from inside a parked car.
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