There are legitimate differences, not just in terms of policy, but also in terms of world view, between the core constituencies of the Republican and Democratic parties. As you move up the ranks to the national committees, you find the narrower distinction between “left liberal” and “right liberal,” as ably adumbrated by Mark Richardson.
By the time you get to the very top, with the Bush and Clinton families, there is essentially no distinction at all. You won’t get much clearer evidence of that than young Barbara Bush’s endorsement of Hillary Clinton for the next Democratic presidential nominee. This comes on top of Barbara Bush Jr.’s involvement with the Clinton Global Initiative and Michelle Obama’s “Let’s Move” group.
You’d think that there would be enough Republican organizations for Barbara Bush to be involved with, and that she might just show some loyalty to the party that nominated two members of her family. Even if she didn’t feel any loyalty to the Republican party, she might at least be supportive of it in case Jeb Bush eventually decides to run for the presidency. However, not even crass considerations like that appear able to sway her.