Donald Trump experiences his worst day in the race as some polls no longer show him as the frontrunner. Tune into World News Tonight with David Muir .\r<br>\r<br>Donald Trump experiences his worst day in the race as some polls no longer show him as the frontrunner. Donald Trump continues to lead Republican U.S. .\r<br>\r<br>Donald Trump continues to lead Republican U.S. presidential candidates among likely GOP caucusgoers in Iowa, but retired neurosurgeon Dr. Ben Carson is .\r<br>\r<br>Make ABC News your daily news outlet for breaking national and world news, broadcast video coverage, and exclusive interviews that will help you stay up to .
