"He insulted my grandfather and attacked the very qualities — loyalty, bravery and selflessness — that he and countless other POWs embody," she added.

McCain wrote that Trump never won her over during his candidacy, calling him a "misogynist," "racist" and "demagogue," citing his remarks on women and minorities throughout his campaign.

She said she decided to support Clinton during the Democratic National Convention this week, where Democratic heavy-hitters touched on themes of American exceptionalism in an effort to win over reticent Republicans.

"The party I grew up in, the party I want to work in and change and push to be more inclusive, betrayed me and countless others," McCain wrote.