千里之堤，溃于蚁穴

one ant-hole may cause the collapse of a thousand-li dyke;a small leak will sink a huge ship; slight negligence may lead to great disaster
～，如…访台，将为台在…政府恪守的一个中国政策上打开缺口，将会产生严重后果和恶劣影响。As a Chinese saying goes，“A single ant-hole may cause the collapse of a thousandli dyke”.If… should visit Taiwan,it would enable Taiwan to sabotage the one China policy pursued by the…government and cause grave consequences and pernicious repercussions.