Politico: The president pledged to put his full weight behind a 2011 push for legislation that would wean the country off fossil fuels. Obama has taken heat from environmental groups for being unable to pass such legislation during his first two years in office. Obama also challenged liberals who are threatening to sit out the vote, saying he’ll need a big turnout five weeks from now if his policies are going to have any chance at success, including energy and climate. “The idea that …