Washington — Louisianans are voting Saturday in the state's Republican Senate runoff, as two candidates vie to replace Sen. Bill Cassidy, who did not receive enough votes in the primary to advance.   Rep. Julia Letlow and Louisiana state Treasurer John Fleming are facing off in the runoff, after no candidate secured 50% of the vote in last month's primary. At the time, Letlow came in first with more than 44% of the vote, while Fleming secured 28%. Cassidy, the GOP incumbent, came in third.