> The fault lines are already there: Liberals like Obama favor massive spending on ambitious social programs. Civil libertarians -- there seem to be quite a few here on HN -- want a reduced government presence and an expansion of individual rights. Centrists like Bill Clinton aren't opposed to overseas military intervention and support balanced budgets and the reform of social programs.
The fact that you refer to "massive spending on ambitious social programs" as a "liberal" idea, and distinguish "liberal" from "civil libertarian", already kind of belies the issue. In most countries, there's a distinction between liberals and leftists. A liberal is someone who favores reduced government presence, strong individual rights, and markets--kind of like US libertarians, except far less extreme. An example would be the UK Liberal Democrats. Leftists are the ones who favor workers and social programs, and have strong ties to labor unions, like the UK Labour party. Conservatives are like conservatives here except, again, less extreme. Conservative parties in countries like the UK or Canada still tend to favor universal healthcare, for instance.
As we've seen in the UK, liberals can and do coalition with either side. You tend to see similar things in the US, though--whatever party holds the Presidency usually wants to expand executive powers at the expense of civil liberties, which leads the other party to reflexively take liberal stances. For instance, the Clinton administration's efforts against cryptography and gun ownership led to Republican opposition on both fronts, while the Bush administration's advancement of warrantless wiretapping and indefinite detention led to strong Democratic opposition that disappeared as soon as a Democratic president was pushing for the same things.
Actually, the problem is that there is no coherent liberal agenda in America. Philosophically, gay marriage, gun rights, opposition to warrantless wiretapping, and marijuana legalization are all liberal viewpoints, but out of the current political parties, each position is respectively supported by Democrats, Republicans, no one, and a minority on both sides.