The Howard Jarvis Taxpayers Assn. says it endorses only issues in line with its fiscal conservatism. But it is also the face of numerous campaigns bankrolled by less popular interests.SACRAMENTO — Perhaps the most formidable opponent Gov. Jerry Brown faces in his crusade for tax hikes has been the Howard Jarvis Taxpayers Assn., the group that fought property taxes the first time he was governor and has since become famous as the voice of middle-class Californians beleaguered by costly big government.