Jeb Bush’s increasingly serious and public examination of a run for president has shaken the ranks of establishment Republican donors and fund-raisers who had planned to back Gov. Chris Christie of New Jersey in 2016, forcing many of them to rethink their allegiance to the embattled governor.

In private conversations that are now seeping into public view, some of them are signaling to Mr. Christie’s camp that, should Mr. Bush enter the race, their first loyalty would be to him, not to Mr. Christie, according to interviews with more than two dozen of them.

Many of those who, because of geography and personal ties, were expected to line up behind Mr. Christie say they now feel torn. And it is clear that Mr. Christie’s recent troubles, especially the George Washington Bridge scandal, are adding to the allure of Mr. Bush, a former Florida governor.

Lawrence E. Bathgate II, a former finance chairman of the Republican National Committee and a major donor in New Jersey, said he dreaded the prospect of having to choose between the two men, calling it “a fraught decision.”