Another Republican national security professional is publicly backing Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE for president instead of Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE.

Kori Schake, a research fellow at Stanford's Hoover Institute and a former senior defense and National Security Council official under former president George W. Bush, will be casting her vote for Clinton this fall, her sister tweeted on Saturday.

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"For the first time ever @KoriSchake and I are casting our vote for the same candidate -- She is voting for @HillaryClinton," her sister, Kristina Schake, a Democrat and deputy communications director for the Clinton campaign, tweeted.

For the first time ever @KoriSchake and I are casting our vote for the same candidate -- She is voting for @HillaryClinton. — Kristina Schake (@kristina_schake) June 25, 2016

Kori Schake served as director for defense strategy and requirements on the National Security Council during Bush's first term, deputy director for policy planning at the State Department during his second term, and as a senior policy adviser for Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE's (R-Ariz.) 2008 presidential campaign.

Schake told The Hill it wasn't a hard decision for her to make.

"I was going to write in my nephew, or write in [former Indiana Gov.] Mitch Daniels, [but] I am actually worried Trump could get elected," she said.

"Watching the British debate on the referendum about the EU, I don't want to wake up the morning after the election, and think, 'Oh my God, all of us who could have prevented this didn't do enough.'"

Schake signed a March letter from 121 Republican national security professionals who said they would not be voting for Trump.

Several of those signatories have said they would vote for Clinton, including Robert Kagan and Peter Mansoor.

Hank Paulson, a Treasury secretary under Bush, and Dick Armitage, deputy secretary of State under Bush, recently said they would back Clinton. Republican columnist George Will recently announced he was leaving the GOP because of Trump but did not say whether he would cast a vote for Clinton.

Schake said many of her Republican national security colleagues will also hold their nose and quietly vote for Clinton.

One of the organizers of the letter, Bryan McGrath, the deputy director for the Center for American Seapower at the Hudson Institute, expressed disappointment over Kori Schake's decision. He has vowed not to support Trump but also not to support Clinton either.

"I hate that it has come to this," he tweeted Sunday about her decision. "I understand .@KoriSchake decision, but cannot support it. There are other paths."

I hate that it has come to this. I understand .@KoriSchake decision, but cannot support it. There are other paths. https://t.co/XP8KOIpahh — Bryan McGrath (@ConsWahoo) June 26, 2016

McGrath told The Hill he is planning to write in Sen. Marco Rubio Marco Antonio RubioSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll MORE (R-Fla.), who ran unsuccessfully for the nomination, and has recently decided to run again for the Senate.

"What's important is that the right of center national security community has to get its act together while it's in the wilderness for a few years, and the way to do that is not to make common cause with Hillary Clinton and the left," he said.

Still, McGrath added, "Kori's a friend of mine, and I respect her decision, and I expect there will be more making the same decision that she has."

This story was updated at 1:26 p.m.