The endorsement represents another step in McCain's tightening grip on the Republican presidential nomination. The officials spoke Friday on condition of anonymity because the formal announcement is next week.
Since he took a commanding lead in the delegate count, the Arizona senator has been working to solidify his support from the Republican base. On Thursday, he picked up the endorsement of his one-time chief rival, former Massachusetts Gov. Mitt Romney. The endorsement of the former president and patriarch of the Bush family also could help with party stalwarts.
His son, the current President Bush, has not yet formally endorsed McCain because former Arkansas Gov. Mike Huckabee in still in the race, but he strongly signaled his support last weekend in a cable television interview.
The president told "Fox News Sunday" that McCain is a "true conservative" but may have to work harder to convince other conservatives that he is one of their own.
McCain "is very strong on national defense," Bush said in the television interview. "He is tough fiscally. He believes the tax cuts ought to be permanent. He is pro-life. His principles are sound and solid as far as I'm concerned."
When asked about criticism of McCain by conservative commentators Rush Limbaugh and Ann Coulter, the president said, "I think that if John is the nominee, he has got some convincing to do to convince people that he is a solid conservative and I'll be glad to help him if he is the nominee."
Bush said some of the criticism of McCain was the result of "probably, some personal animosity toward me. You can't please all the people all the time."
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