While the calls to dump Trump have continued since the 2012 Republican nominee Mitt Romney raised the alarm about the brash billionaire's candidacy early this year, it is unclear whether the trickle of defections to the Clinton camp will become a flood.
But a nightmare 24-hour period for the embattled Republican nominee -- he refused to end his public feud with the parents of a fallen Muslim American soldier, declined to back House Speaker Paul Ryan's re-election bid and used crass language while accepting a supporter's Purple Heart as a gift -- has laid bare the concerns Republicans have with Trump.
Whitman, herself a billionaire and leading Republican fundraiser, is chief executive of Hewlett Packard Enterprise and used to be CEO of eBay. She spent some USD 140 million of her own money in an unsuccessful run for California governor in 2010.
Her defection comes amid deep concerns at the top of the Grand Old Party about Trump, after a seemingly never-ending stream of controversies that include the nominee urging Russia last week to meddle in the US election by hacking into Clinton's emails.
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Whitman joins other key Republicans who have announced their backing for Clinton, the former secretary of state.
Richard Armitage, who served as deputy secretary of state for George W. Bush and deputy secretary of defense under Ronald Reagan, defected in June.
Trump "doesn't appear to be a Republican, he doesn't appear to want to learn about issues," Armitage told Politico. "So, I'm going to vote for Mrs. Clinton."