BRASILIA (Reuters) - Former Brazilian senator and environmental minister Marina Silva said on Saturday that she would seek her party s nomination to run for president next year.  Silva announced her plans at a meeting of her Sustainability Network Party, or REDE, which would officially nominate her at its national convention in April. The 59-year-old environmentalist, born into a rubber-tapping community in the Amazon rainforest, was minister under former President Luiz Inacio Lula da Silva. She has run in the previous two presidential elections but never made it to a second-round vote.  Silva said she would seek her party s nomination because  Brazilians want a country free of corruption  and that she had the ethical bearings to deliver on that.  Supporters have praised her as Brazil s most principled politician, which could play a large role in the next election. Since early 2014, Brazilian prosecutors and police have carried out an unprecedented anti-corruption drive that has revealed political graft touching every major party, but not Silva s. Silva has remained committed to fiscal responsibility, inflation targeting and a floating exchange rate, the so-called tripod of economic policies that gave Brazil stability after a period of rampant inflation and erratic growth in the 1990s.   In the latest election survey conducted by the respected Datafolha polling institute and published Saturday on the Folha de S.Paulo newspaper s website, Silva ranked third, with 10 percent of respondents saying they would vote for her.  But that poll included Lula as a possible candidate.  Lula was convicted in a corruption trial in July. If a higher court upholds that conviction before the October election, he could not run.  Without Lula, the Datafolha poll had Silva trailing right-wing candidate Jair Bolsonaro, a federal lawmaker who won 21 percent approval, compared with her 16 percent. Datafolha interviewed 2,765 voters across Brazil on Nov. 29-30. The poll had a two-percentage-point margin of error.  In the 2010 election, Silva, then with the Green Party, took a surprising 20 percent of the first-round vote, but it was not enough to push her into a runoff with eventual winner Dilma Rousseff.  In 2014, she ran for vice president on the Brazilian Socialist Party ticket headed by Eduardo Campos, who died when his campaign plane crashed just two months before the election. Silva took his spot and won 21 percent in the first round, again missing out on a runoff.  