WHILE beloved NANCY REAGAN’s health rapidly deteriorates, the former first lady’s children are already fighting over her $15 million estate.

And the battle lines have been drawn with Nancy’s biological kids Ron, 54, and Patti Davis, 60, on one side versus their dad’s 67-year-old adopted son Michael on the other.

Insiders say the two are begging Nancy, 91, to completely freeze Michael out of her will!

“BOTH Ron and Patti can’t  stand Michael,” revealed a family friend. “They feel that when Nancy dies, Michael doesn’t deserve a dime of the Reagan estate.”

That’s because the late actress Jane Wyman was Michael’s mom, not Nancy.

“But Michael believes he deserves something since all the money comes from his dad’s property,” continued the friend.

“Meanwhile, he hates Ron and Patti with a passion. They never supported their dad politically, as he did.”

Michael is especially bitter at Ron for publishing “My Father at 100” in 2011. The conservative radio talk-show host took to his Twitter page to denounce the book and its claim that the elder Reagan may have had Alzheimer’s when he was president.

“My brother seems to want to sell out his father to sell books…my father did not suffer from Alzheimer’s in the 1980s,” Michael furiously tweeted.

He later added: “Ron, my brother, was an embarrassment to my father when he was alive and today (after the book’s publication) he became an embarrassment to his mother.”

But Ron insists Nancy loved the book and that she felt Michael was just being “mean” to him.

The family war is especially sad because it’s filling Nancy’s final days with ugly squabbling.

ALL the details are in the latest ENQUIRER!