Well, in 1912, Teddy Roosevelt ran as a 3rd party candidate, leaving the Republican party. The split in the Republican vote helped elect Woodrow Wilson. It’s hard to imagine Hillary going independent would get her elected, but it’s easy to see the split Dem. vote helping elect McCain.
I don’t know; in a three-way race between her, McCain of B. Hussein Osama-Obama, she just might get a plurality. A plurality put her hubby in the White House two times.