I’ve thought much the same thing for a while, Andrew, although I emphasize the tentative Republicans’ libertarianism, rather than what you call “managerialism.” It’s unfortunate that Terri Schiavo has become the excuse of the week for libertarians to stomp their feet about the “theocrats” with whom they have to share a coalition.
As I see it, there are two possibilities when it comes to the libertarian faction of Republicans shifting over to the Democrats:

1. The Democrats could learn how to compromise with them on small-government issues.
2. The Democrats could run their party into the ground so dramatically that the libertarians essentially take it over.

Number 1 would be the most expedient route — with number 2 taking decades, I would think — but it doesn’t look likely. Whenever Democrats want to appeal to a broader base, they move toward social conservatives, not governmental conservatives. (Witness Hillary.)
The Schiavo case, especially considering that the legislative and executive branches dealt with it by handing it back to the judiciary for another chance to bail them out of a tough political spot (sort of like the legislature’s initial reaction to Goodridge in Massachusetts), makes me wonder whether those who trumpet the possibility of a “Republican crack-up” don’t have it backwards. The common line is that social conservatives are busily enacting their moral policies, but what’s actually happening is that they’re being thwarted in doing so by the representatives to whom they look for influence. Moreover, since their involvement in government and politics is driven by something other than government and political theory, they may be more inclined to disengage from the coalition (whether that means disengagement from the entire process or just from the party).
I wonder: would libertarians have been more incensed by decisive legislative/executive action in the Schiavo case than the social conservatives will be by the ineffective show put on last weekend? I don’t think so. I do think, however, that politicians and libertarian Republicans both should be wary of assuming there to exist an immovable “base.” One too many monuments torn down, one too many lives not saved, one too many social policies rewritten in the courts, and social conservatives may decide that their time is better spent in the socio-cultural arena. That’s what libertarians claim to want, but they may not like the political consequences.