But the president insisted that's no reason "she can't be a great Supreme Court justice."
Interviewed on NBC's "Today" show during, the president was asked about critics from the right wing of his own party who've called Miers unqualified and a disappointment.
"My answer is Harriet Miers is going to be confirmed and people will get to see why I put her on the bench," he said.
On the same program, first lady Laura Bush was asked if she shared her husband's conviction. "Absolutely. Absolutely," she said.
"She's very deliberate and thoughtful and will bring dignity to wherever she goes, but certainly to the Supreme Court. She'll be really excellent," Mrs. Bush said.
Asked if she believed some of the criticism reflected possible sexism, she responded: "I think that's possible."
Meanwhile, some newly released letters show a very warm relationship between Mr. Bush and Miers, his White House counsel and former personal attorney.
Miers told Mr. Bush in a 1997 birthday card that he was "the best governor ever" and, in a separate note to her boss, said she hoped his twin daughters recognize their parents are "cool."
The correspondence with Mr. Bush was among thousands of pages of records released Monday by the Texas state archives from Miers' record as chair of the Texas Lottery Commission from 1995-2000.
A transcript of her confirmation hearing before the state Senate in 1995 indicated that lawmakers asked Miers, an attorney, only a few questions and that her nomination by Mr. Bush was generally unopposed.
Miers and Mr. Bush exchanged several birthday notes and general well-wishes during her lottery tenure.
In 1997, Miers sent him a belated birthday card featuring a sad-looking dog and the note: "Dear Governor GWB, You are the best Governor ever — deserving of great respect!" She added, "At least for thirty days — you are not younger than me."
Mr. Bush's birthdate is July 6, 1946; Miers' is Aug. 10, 1945.
Mr. Bush wrote back to wish Miers a happy 52nd birthday, telling her that he appreciated her friendship and to "never hold back your sage advice." He ended with a postscript: "No more public scatology."
That October, Miers wrote Mr. Bush a note saying she hopes his twins, Jenna and Barbara, recognize they have "cool" parents.
Mr. Bush told Miers in a birthday note in 2000: "Have a great life!"