HARTFORD, Conn. — One of America’s largest troves of political and campaign memorabilia is headed for the auction block, spurring protests from some who do not want to see it divided up and sold to private collectors.The University of Hartford plans to hire an auction house and sell off more than 70,000 items, many of them donated by the late J. Doyle DeWitt, a former chairman of The Travelers Cos. who spent decades amassing letters from presidents, campaign posters and pins and advertisements dating to the …