The afformentioned tour was the "America Goes Back to School" bus tour of which Tipper Gore was the co-chair. It was a program designed by the US Secretary of Education to support local, state and community involvement in school life.
www.ed.gov/offices/OPA/success/index.html
She was co-chair in 1999 also, along with PTA President Ginny Markell and Patti LaBelle, the first year (not an election year) that the bus tour took place. (Chairs remained the same both years)
www.ed.gov/PressReleases/07-1999/agbts.html
I have no doubt that if Laura Bush wanted to be on this committee (as a volunteer like those mentioned above) that she would be accepted.
Perhaps you have forgotten that before Tipper Gore was the wife of a VP and Presidential candidate, she heads "Parent's Music Resource Center", one of the groups involved in putting warning labels on music. (starting back in 1985) As part of that group, she worked with the PTA. She was keynote speaker in 1997 also. There is a notable history of her involvement, as a PARENT.
You reference Laura Bush,(former school librarian and a republican)Mrs. Bush was a guest speaker at a PTA event in 1996.
We have had the Governor speak at our state conventions (he's a republican) our senators (democrat and republican)
The National PTA legislative conference includes keynote speakers, Dr. Julianne Malveaux and Deborah Perry Piscione (a popular republican commentator and former appointee for Pres George Bush Sr.)
It's easy to find division when you are looking for it.