(CNN) - Mitt Romney headlines the Republican Party of New Hampshire's convention later this month, the fourth time over the past two months that the past and possible future GOP presidential hopeful is helping out party officials in the crucial early primary state. The former Massachusetts governor will serve as keynote speaker at the state party convention, being held September 25 in Concord. Romney's role was first reported by the New Hampshire Union Leader and confirmed by state party officials. Romney was the main attraction at an August 5 fundraiser in New Hampshire to benefit the House Republican Victory PAC,...