[Donald Trump]Joe Raedle/Getty Images Some wealthy Republican donors appear to be having a tough time getting behind Donald Trump's presidential campaign. Despite the fact the presumptive GOP nominee has obliterated his challengers during a raucous primary season, garnered a handful of endorsements from Washington, and enjoys the enthusiastic support of millions of voters, portions of the donor class are not sold. Trump has made a point of highlighting his partially self-funded campaign over the past year, but now as the businessman prepares to raise $1 billion for the general election, Trump is on the hunt for some big-time cash. But, according to a report from The New York Times, a handful of deep-pocketed donors have suggested that they will ignore Trump's calls. Among them, New York investor Paul Singer, "who has spent at least $28 million for national Republicans since the 2012 election," the Times noted. Singer had thrown his support behind Florida Sen. Marco Rubio in October. TD Ameritrade founder Joe Ricketts and hedge fund manager William Oberndorf have also disavowed Trump, according to the Times. "If it is Trump vs. Clinton, I will be voting for Hillary," Oberndorf said. The collective trepidation stems in part from Trump's erratic and ever-changing policy positions over the course of his campaign — many of which have prompted his rivals to label him as "unhinged" and unfit for the presidency. NOW WATCH: FORMER GREEK FINANCE MINISTER: Why Hillary Clinton is a 'dangerous person'