So You Think You're Smart? Take 2

My first "So You Think You're Smart?" puzzle was solved fast -- too fast! Here's the next one: let's see how long it takes to solve this one:

An interesting fact is that three U.S. presidents had mothers named "Rose". The three were: John Kennedy, Bill Clinton, and Woodrow Wilson.

Now, we know that mothers see their sons in the kindest light possible. Yet, in spite of this, one of these mothers (alive at the time of their son's election) didn't vote for their own son!

Question: which Rose didn't vote for her baby -- and why?