Why China has become a hot topic in the US presidential campaign

I am from china. I have always had a question, why every time the U. S. presidential campaign, China will become the topic，which must be talk ? But the attitude of the president of the United States is not what it is like to be in the United States of America when the president is the president of the United States of America.（shinese version：我是来自中国。我一直有一个疑问，为什么每次美国总统竞选中，中国都会成为各总统候选人必须大谈特谈的话题？但是，每当总统候选人继任美国总统之后，对中国的态度却不是竞选时候对美国民众所承诺的那样？）