By Daniel Trotta

CLEVELAND (Reuters) - A handful of donors to the Republican National Convention have withdrawn pledges and fund-raising has stalled during Donald Trump's polarizing bid for the presidency, leading organizers to ask casino mogul Sheldon Adelson for an urgent donation.

The Cleveland 2016 Host Committee said in a statement on Friday it had approached Adelson, a generous party benefactor, while also acknowledging its fund-raising has come to a virtual standstill, leaving a $6 million shortfall.

"Negative publicity around our potential nominee resulted in a considerable number of pledges backing out from their commitments," the Host Committee said in a letter it sent to Adelson, according to the political news website Politico, which said it had obtained the letter.

The Host Committee on Friday confirmed such a letter was sent to Adelson and his wife, Miriam Adelson, but it downgraded the withdrawn pledges from "considerable" to "a handful." It said the hastily sent original letter "mischaracterized" the status of some donations. Some companies identified in the letter as having backed out said on Friday they had never made such pledges.

"Unfortunately, this letter (to Adelson) was not reviewed nor authorized by the Host Committee chairpersons, and it mischaracterized certain donations from individuals and corporations," the statement said.

"Some of what were referred to as pledges were actually expectations based on pledges made to previous conventions, while a handful had been withdrawn from the Host Committee for 2016 Republican National Convention," it said.

The committee said it had apologized to the Adelsons' staff.

Host Committee officials on Friday declined to comment beyond the official statement, leaving open the question of which donors reneged. Donations will be disclosed after the convention in U.S. Federal Election Commission filings.

The statement, from Host Committee Chief Executive David Gilbert, also said the committee had raised $58.25 million to date, about $6 million short of its goal of $64 million to cover expenses for the four-day convention that starts Monday in Cleveland, where Trump is expected to receive the Republican presidential nomination.

A month ago, a committee spokeswoman said $57.5 million had been raised.

The Politico report, citing the letter, said David Koch, like Adelson a wealthy donor to conservative causes, and Coca-Cola <COKE.O> each withdrew a pledge of $1 million. Koch and Coke representatives told the Wall Street Journal they never made such pledges. Coke previously said it donated $75,000 last year.

Visa, cited in the letter as having backed out of a $100,000 pledge, said in a statement it never made a pledge.

FedEx <FDX.N> and BP <BP.L> were named in the letter as withdrawing support but also told the Journal they had not made the stated commitments.

Other marquee companies were also named but did not respond to Reuters queries after business hours on Friday. Representatives for the Adelsons could not immediately be reached for comment.

Last month activists disturbed by Trump's campaign and led by the political arm of California-based advocacy group Color of Change launched an effort to try to shame companies into dropping sponsorships.

It was unclear if that effort, which targeted more than 30 companies, had an impact.

(Reporting by Daniel Trotta; Additional reporting by David Greising; Editing by Andrew Hay)