"People may always have a difference of opinion on certain lifestyles, but championing a position that wants to treat people unequally isn't just un-Republican. At its fundamental core, it's un-American," she wrote.
The mini maverick declared her support for same-sex marriage and urged more Republicans to embrace the gay and lesbian community -- a move she thinks is "vital to the future success of the party."
"Sometimes, gay Republicans are regarded in the same vein as 'Jews for Hitler,' but my father garnered nearly 28 percent of the gay vote in the last election," McCain wrote.
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The young blogger warned against conservative rhetoric that often sends an unwelcoming message to gay voters. She also criticized her father's former foe Barack Obama.
"President Obama, for instance, is also against gay marriage -- a dirty little secret many of my gay friends were shocked to discover. But you'd never know it because he always 'sounds' so inclusive," she wrote.
McCain will be speaking about gay rights at the Log Cabin Republicans' national convention this week.