The White House, unaware of historic norms, had been on track to give more than the usual 30 percent of ambassadorial jobs to political appointees until objections from career diplomats forced it to reconsider, administration officials say.

As a result of the reversal, some donors to President Obama’s election campaign — as well as senior advisers and other supporters of the campaigns of Vice President Joseph R. Biden Jr. and Secretary of State Hillary Rodham Clinton — are likely to find their hopes of being rewarded with an embassy dashed. “The White House has come around, and we truly expect that, at the end of the process, the balance will be within historical norms,” said one senior administration official who asked not to be named because he was discussing internal deliberations.