Chair Meg Whitman announced via Facebook late Tuesday night that she will be supporting Democratic candidate Hillary Clinton's run for the presidency.
Campaign officials said more than half of the July contributions came from first-time givers, potentially providing her campaign with a fresh pool of donors to tap for multiple contributions in the weeks ahead.
Whitman argued that Clinton's temperament, global experience and commitment to America's bedrock national values make her the far better choice for president in 2016.
Whitman's endorsement follows on the heels of a similar endorsement from Republican congressman Richard Hanna, who called Trump "deeply flawed".
According to a CBS News poll on Monday, Mrs Clinton leads Trump by six percent, having 46 percent of support compared to 39 percent Trump secured.
"While I disagree with her on many issues, I will vote for Mrs. Clinton", he wrote.
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Whitman told the Times that Trump is a "dishonest demagogue" and that electing him to the White House would lead the United States "on a very risky journey".
Whitman's decision comes at an inopportune time for Republican supporters as Trump has faced backlash for his controversial comments regarding sexual harassment and an ongoing feud with the family of US fallen soldier Humayun Khan.
While Whitman maintains that she holds very different viewpoints on many issues than Clinton, she stated on Friday in an interview with CNBC that she will not be voting for Trump.
The poll showed Clinton with a 10-point lead over Trump, with 49% of 1,022 registered voters saying that they back the Democratic nominee.
She told the Times that Republicans need "to put country first before party" and said that she would remain a member of the GOP.
Whitman was a top fundraiser for Mitt Romney's 2008 presidential campaign and chaired New Jersey governor Chris Christie's finance team this year until he pulled out of the race.