When Bush wanted more testing and accountability from schools as governor of Texas, it was Spellings who oversaw the plan and gained support from business executives and lawmakers.
When Bush made student achievement and school choice his first domestic priority as president, Spellings led the negotiations with congressional leaders. The deal that emerged, dubbed No Child Left Behind (search), is the largest federal overhaul of education in a generation.
Now, as schools scramble to keep up with the law and Bush seeks to expand it, he has nominated his confidante to be education secretary.
"She was his first policy person, ever, in politics," said Sandy Kress, a former Bush education adviser who worked closely with Spellings in the White House. "How many Cabinet members were there when he first started during his run for governor?"
If departing Secretary Rod Paige (search), who came to Washington as a superintendent from Houston, was the reform-minded outsider, then Spellings is the loyal insider. She was Bush's chief education adviser in Texas and his domestic policy chief inside the White House.
Described as approachable and practical, Spellings even doles out rejection well. She once turned down a date from Karl Rove when both were single in the early 1980s. Rove, now Bush's election architect, later introduced Spellings to Bush.
At her Senate confirmation hearing Thursday, Spellings is likely to be pushed on issues from preschool to college. She is expected to easily win confirmation.
If so, Spellings, 47, would become the nation's eighth education secretary and the first woman to hold the job in more than 20 years. She also would be the first secretary in recent history to have school-aged children. Her daughter Mary, 17, attends a Catholic high school and her daughter Grace, 12, goes to a public middle school.
When Spellings accepted the nomination, she choked up briefly in describing her belief in America's schools. Bush gently patted her back in support.
Their relationship could mean more power for the education secretary. Bush admired Paige's hands-on experience and passion for empowering poor and minority kids, but political observers say Paige was given little leeway to lead the debate.
As Bush himself put it, there is no one he trusts more on education than Spellings.
That could allow her to "really shape the thinking of the administration," said Christopher Cross, an assistant education secretary under Bush's father. "She'll cut out the middle man. Clearly, Paige became the middle man between the White House and Congress and the education community."
Spellings also would gain the power to enforce the very law she helped put together. Under No Child Left Behind, students must show gains regardless of race or wealth, and schools that receive federal poverty aid but fail to make yearly progress face penalties.
"She has deep subject-matter expertise. That will go a long way toward making her life easier and possibly re-energizing the department," said Paul Light, a New York University professor who runs the Presidential Appointee Initiative (search) for The Brookings Institution.
Yet Spellings would face a diverse range of critics who support the law but want regulatory changes and better guidance from the department. The agency also oversees many other matters, from college aid to enforcement of civil rights.
Spellings is known for championing early reading and regular testing, but many Republicans want to hear more from her about private-school vouchers. Paige won praise from voucher advocates by using his bully pulpit to promote school choice.
Meanwhile, senators of both parties say they've found Spellings to be fair and focused.
Sen. Edward Kennedy of Massachusetts, the top Democrat on the education committee, calls Spellings a principled leader who "has earned strong bipartisan respect in Congress."
She had the same fair reputation in Texas, said an occasional adversary, Jay Levin, chief lobbyist for the Texas State Teachers Association (search). On teacher's rights and contracts, Levin was at odds with Spellings when she lobbied for the Texas School Boards Association.
The National Education Association (search), a teachers union with 2.7 million members, sees Spellings as the administration's chance to make amends with educators. Paige enraged the NEA by calling it a "terrorist organization" and for opposing aspects of No Child Left Behind.
Spellings attended public schools and the University of Houston. She was a divorced, single mother when she came with Bush to Washington and has since married Robert Spellings, a lawyer with long ties to Texas politics.