Montserrat Gonzalez, 60, shot Isabel Carrasco - the conservative Popular Party (PP) leader of the provincial government - in the back on the afternoon of May 12th, 2014 as she walked on a pedestrian footbridge in the northern city of Leon, prosecutors and witnesses say.
With her face covered by a scarf and sunglasses, Gonzalez shot Carrasco two more times in the head before walking away with her daughter who was nearby, according to prosecutors.
A retired police officer who happened to be on the footbridge when the killing occured trailed the pair and called police who arrested Gonzalez and her daughter, Triana Martinez.
The man also saw how the pair left the gun used in the killing in a car belonging to a policewoman with the city of Leon, Raquel Gago, who was also arrested.
Gonzalez told a court in Leon on the opening day of the trial of the three women that she killed Carrasco as revenge for the way her daughter had been treated by her.
Her daughter's temporary contract with the Leon provincial council ended in 2011 and another candidate was chosen to replace her.
Gonzalez told the court that her daughter was let go from her job because she refused to have sex with Carrasco, who had led the provincial government of Leon since 2007.
Asked if she regretted killing the politician, Gonzalez told the court: "No. I would be lying if I said otherwise."
"She would have continued to make life impossible" for my daughter, Gonzalez added.
Public prosecutors have asked for Gonzalez, her daughter and the policewoman whose car was used to hide the weapon used in the killing to each be sentenced to 23 years behind bars.
Carrasco's murder shocked a country unused to such acts since the Basque separatists ETA announced an end to violence. Numerous PP officials were assassinated in the 1990s and early 2000s in killings blamed on ETA, which declared a definitive end to violence in October 2011.