The Sierra Club gave its formal endorsement to Al Gore today, with the group’s president, Robert Cox, saying that if Gore were elected, he would be “the most pro-environment president in our history.” Gore accepted the endorsement by saying, “It is an honor that I will seek to live up to every day.” The Gore campaign is working hard to draw attention to the announcement, which it hopes will help shore up support among enviros considering a vote for Ralph Nader, the Green Party candidate. Twelve of the Sierra Club’s 15 board members voted to endorse Gore on Saturday, with only one favoring Nader.