Republican Presidential candidates ripped President Obama on Tuesday for what they called his stubborn insistence on linking gun violence with guns. In campaign stops across Iowa and New Hampshire, the G.O.P. hopefuls pounded the President for irrationally concluding that guns have played a role in the nation’s epidemic of mass shootings. “How any reasonable person could look at gun violence and say that guns are involved is beyond me,” said Texas Senator Ted Cruz. “And yet, somehow, President Obama always finds a way.”