The shift away from the broadcast television buys that had been the groups’ main role in past presidential campaigns is among the most significant developments in outside political spending since the Supreme Court’s 2010Citizens United decision, which paved the way for super PACs. Originally conceived as a vehicle to raise and spend unlimited money on television, the most expensive part of a White House run, the groups now are seeking to relieve campaigns of much of the vital infrastructure that candidates would otherwise have to assemble and manage themselves.