Monday on the NewsHour, the southeastern U.S. prepares for a possible hit from Hurricane Dorian. Plus: Hurricane Dorian slams the Bahamas, where rescuers have been unable to get to many of the stranded, a conversation with former Defense Sec. James Mattis, another mass shooting in Texas, Brexit drama in the UK, 2020 Democrats talk gun laws and Politics Monday with Tamara Keith and Amy Walter.