If Republicans were feeling evil in Louisiana, they'd be smart to encourage a couple of Democrats to run in this race, particularly an African-American who may be able to peel some votes from Landrieu. Anything possible to keep her under 50%.
They used to do that back in the Lee Atwater days. The problem is that today the Democrats are the far more disciplined, orderly party that keeps their members and candidates in line and it's the Republicans who are vulnerable to meddling by activists and extremists. Will Rogers' quip that he "doesn't belong to an organized political group - I belong to the Democratic Party" is better suited to the Republicans these days.
I still think this is Mary's race to lose. The fact is that money talks, even more so in this day and age, and the oil companies and the lumber companies and everybody else have already invested a lot of money in her over the years. She has experience and seniority. She's been good to them. They'd be foolish to throw her over for an insubstantial Republican like Bill Cassidy.