New Jersey Gov. Chris Christie addresses the media Thursday, May 26, 2011 in Trenton, N.J., as he announces that New Jersey will withdraw from a 10-state regional greenhouse gas reduction program by the end of the year, saying the program is ineffective at combating global warming. The announcement thrilled conservatives, who have been dogging governors in Northeast states to abandon the effort to limit greenhouse gas emissions by having polluters pay for their fossil fuel output. (AP Photo/Mel Evans) The Associated PressWASHINGTON (AP) — For Republican presidential contenders who once supported combatting global warming, the race is heating up.Most of the GOP White House hopefuls have stepped back from their previous positions on global warming. They're catering to an activist right wing that questions the science of climate change and disdains big government.When they were governors, Tim Pawlenty, Jon Huntsman and Mitt Romney all supported regional plans to curb greenhouse gases. Romney changed his position in 2005. Pawlenty has apologized for it. Huntsman has said the time isn't right because of the economy.Newt Gingrich once called for action on climate change. Now, conservatives are demanding an apology from him.On Thursday, New Jersey Gov. Chris Christie said he's pulling his state out of a regional plan to reduce greenhouse gases.