"Her story - which is true - needs to be told," she said of her daughter, Donna Joy.
The U.S. Senate on Thursday defeated two alternative measures introduced by Democrats that would have banned abortions past the point where the fetus can live outside the womb. Republicans said the proposals had too many loopholes.
Santorum's legislation would ban the procedure, known medically as intact dilation and extraction, in which a fetus is partially delivered feet first and its skull collapsed. The House of Representatives approved a similar measure last month.
During floor speeches, Santorum displayed blown-up pictures of Donna Joy Watts, 5, both as a baby and today.
Senate rules bar children younger than 6 from the gallery. Santorum asked unanimous consent from his colleagues to waive the rule and let Donna Joy watch his floor speech Wednesday, but Sen. Barbara Boxer, D-Calif., objected.
Her spokesman, David Sandretti, blasted Santorum for using the girl as a "prop."
"In this debate, it was exploitative to use a child," he said.
Sandretti said that Boxer has dozens of examples of women who made painful decisions to have late-term abortions on advice of doctors.
"Sen. Santorum is calling these good women baby killers," he said. "Is that the kind of message we want to send to a 5-year-old girl?"
Watts expressed outrage that her daughter could not watch a debate of great relevance to her own life.
"She said it was inappropriate for her to be there, even though it was her own story," Watts said.
The story of Donna Joy Watts began almost 51/2 years ago, when she was born with multiple rare brain disorders.
One of those disorders, holoprosencephaly, results from incomplete cleavage of the two hemispheres of the brain. Doctors recommended Watts have an abortion, and due to the baby's enlarged head, the procedure would have been a so-called partial-birth abortion.
After her birth, Watts said doctors predicted she would not be able to swallow or walk. Donna Joy was born without the portion of the brain that controls walking.
But time and again, Donna Joy has overcome obstacles. Though still afflicted with many health problems, she eats regular foods and runs around.
For two exhausting days Wednesday and Thursday, Watts found herself in the center of one of the nation's most divisive issues.
In addition to talking with senators, Watts gave interviews to "NBC Nightly News," "Good Morning America" and others.
Santorum spokeswoman Laura Narducci said Donna Joy is a flesh-and-blood example of why the legislation is needed.
"She's living proof," she said. "What we've been saying over and over is that this is procedure is never medically necessary."
Rep. Roscoe Bartlett, R-Md., who introduced the Watts family to Santorum, said he hopes Donna Joy will help convince enough senators to produce a veto-proof majority.
"I hope her presence there will move some people," he said.
Watts said she plans to return to Washington next week when the Senate is expected to vote on the legislation.
"I will back and so will Donna Joy," Watts said.