Given that Landrieu has won two runoffs in the past, I'm guessing that she's not in the hideous shape that some people think she's in.
Louisiana has gone farther to the right, Obama is deeply unpopular, and the race would be heavily nationalized in a runoff, especially if Senate control is up for grabs. She may be a skilled politician, but this is going to be an environment unlike what she has faced in past elections.
But this is already nationalized. How a run off could change her situation, except it is the decisive seat for the senate majority.
Landrieu won 2 run-offs, why couldn't her win a 3th? (except if indeed the decisive seat for the senate majority is her seat).