re: Why would someone vote for Landrieu if he/she also voted against Obama in '12?

Several reasons

Mary helps their businesses by getting them federal contracts

Mary is a female, many women like to vote for other women

Mary has convinced that Cassidy is some kind of rich old tycoon while she is a woman of the people.

Cassidy is not a great candidate, and many Republicans will stay home because he isn't conservative enough lowering the Republican vote; thus helping Mary indirectly.