Sally Bradshaw's departure from the Republican party is considered to be significant given that she has been considered as a Bush family loyalist.
"This is a time when country has to take priority over political parties. Donald Trump cannot be elected president," Bradshaw told the CNN in an interview.
She has worked with the presidential campaign of George H W Bush in 1988.
"This election cycle is a test. As much as I don't want another four years of (President Barack) Obama's policies, I can't look my children in the eye and tell them I voted for Donald Trump. I can't tell them to love their neighbor and treat others the way they wanted to be treated, and then vote for Donald Trump. I won't do it," she said.
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"Every family who loses a loved one in service to our country or who has a family member who serves in the military should be honoured, regardless of their political views. Vets and their family have more than earned the right to those views. Someone with the temperament to be president would understand and respect that," Bradshaw said.
She said she had been considering the move for the past several months.
"I didn't make this decision lightly -- I have worked hard to make our party a place where all would feel welcome. But Trump has taken the Republican party in another direction, and too many Republicans are standing by and looking the other way," Bradshaw said.