Donald Trump on Saturday took shots at Governor Scott Walker of Wisconsin, the only Republican in a field of 16 who is leading the business mogul in Iowa polls. Trump has surged in such polls nationwide, during a campaign in which he has caused controversy over immigration and with a widely condemned attack on the Vietnam war record of Senator John McCain. On Saturday, Trump addressed a rally in Oskaloosa, Iowa from which his campaign had barred the Des Moines Register newspaper, which published a critical editorial about him. At the rally, he repeated several traditionally Democratic talking points in...