Several politicians, including the Rev. Al Sharpton, are slated to appear at a Forest City Ratner rally today in support of the $4 billion Atlantic Yards mega-project. The Downtown Brooklyn rally, paid for by Ratner, will feature live music, including the Brooklyn-based band the Second Step. Some critics say Ratner is gearing up to campaign for more public subsidies for the struggling project. Ratner announced last month that the project’s centerpiece, once called Miss Brooklyn, had been scaled down from 620 to 511 feet.