Although the studio is pushing Mara for Best Supporting, she actually has ore screen time in "Carol" than Cate Blanchett does. There's been a lot of talk about this and some suggestion that voters may list her in Best Actress instead. This has happened before, most recently when Kate Winslet won. So I think there's a decent shot that Rooney Mara could show up on either list.
(Both? Surely the Academy doesn't let that happen...)