(Adds Silva campaign disowning video, sixth paragraph)
BRASILIA, Aug 28 (Reuters) - Brazil's ruling party has asked Google Inc to pull a spliced campaign video from YouTube that gives viewers the impression its leader, Luiz Inacio Lula da Silva, is backing opposition presidential candidate Marina Silva.
Silva, a popular environmentalist, has surged in the polls and is threatening to defeat President Dilma Rousseff in the October elections, unseating Lula's Worker's Party after 12 years in power.
In the video, a cheerful Lula backs the election of "Marina." But the candidate he is referring to is Marina Santana, who is seeking a Senate seat for the state of Goiás. In the false video, Marina Silva's campaign logo has been spliced in instead.
"This video is a blatant fraud," Workers' Party President Rui Falcao said at a news conference. Falcao said the party asked Google, which owns YouTube, to pull the video within 24 hours and identify who posted it.
The Workers' Party is also asking Brazil's electoral court to open a criminal investigation to find out who was responsible and plans a civil lawsuit seeking compensatory damages for harming Lula's image.
Marina Silva's campaign quickly denounced the video as a fraud and called on the authorities to investigate its origin.
Google said it has not received a court order to remove the video. The U.S. company added in a statement that YouTube could be considering its removal if the party used an online reporting tool for legal complaints. (Reporting by Anthony Boadle; Editing by Stephen Eisenhammer, Jonathan Oatis and Cynthia Osterman)