'Almost impossible': Inside Biden's uphill battle to win the backing of key anti-Trump Republicans

• Allies of President Joe Biden are ramping up a charm offensive aimed at getting high-profile Republicans and donors who won't support Donald Trump to back the Democratic president. 
• Several major Republican leaning donors to former presidential primary candidate Nikki Haley's campaign are holding back in backing Biden, at least for now.

Allies of President Joe Biden are ramping up a charm offensive aimed at getting high-profile Republicans and donors who won't support Donald Trump to back the Democratic president. 

But the task is proving difficult, according to people familiar with the efforts. 

As of Thursday, Biden had not yet secured any endorsements from nationally recognized Republican Party officials.

Meanwhile, several major Republican leaning political donors who backed former presidential primary candidate Nikki Haley's campaign are holding back, at least for now, as they make up their minds whether to back Biden.

"Winning campaigns build coalitions, and that is this campaign's north star for every single day between now and November -- earning the support of the voters who will decide this race," Biden campaign spokesman Kevin Munoz told CNBC in an email Wednesday.

For Biden, there is one Republican endorsement that would outshine all the others. But it is a long shot.

A few of Biden's donors who also backed Haley in her primary challenge to Trump have tried to privately encourage her to endorse Biden, according to people with knowledge of the situation.

Like others in this story, they were granted anonymity to describe private conversations.

As of Thursday, Biden and his senior campaign advisers had not connected with Haley or members of her team about a possible endorsement, they explained. And it was