In a truly brilliant piece in the current Washington Monthly magazine, Alan Wolfe explains with precise and persuasive logic Why Conservatives Can't Govern:
The collapse of the Bush presidency, in other words, is not just due to Bush's incompetence (although his administration has been incompetent beyond belief). Nor is it a response to the president's principled lack of intellectual curiosity and pitbull refusal to admit mistakes (although those character flaws are certainly real enough). And the orgy of bribery and special-interest dispensation in Congress is not the result of Tom DeLay's ruthlessness, as impressive a bully as he was. This conservative presidency and Congress imploded, not despite their conservatism, but because of it.

Contemporary conservatism is first and foremost about shrinking the size and reach of the federal government. This mission, let us be clear, is an ideological one. It does not emerge out of an attempt to solve real-world problems, such as managing increasing deficits or finding revenue to pay for entitlements built into the structure of federal legislation. It stems, rather, from the libertarian conviction, repeated endlessly by George W. Bush, that the money government collects in order to carry out its business properly belongs to the people themselves. One thought, and one thought only, guided Bush and his Republican allies since they assumed power in the wake of Bush vs. Gore: taxes must be cut, and the more they are cut--especially in ways benefiting the rich--the better.

But like all politicians, conservatives, once in office, find themselves under constant pressure from constituents to use government to improve their lives. This puts conservatives in the awkward position of managing government agencies whose missions--indeed, whose very existence--they believe to be illegitimate. Contemporary conservatism is a walking contradiction. Unable to shrink government but unwilling to improve it, conservatives attempt to split the difference, expanding government for political gain, but always in ways that validate their disregard for the very thing they are expanding. The end result is not just bigger government, but more incompetent government.
Saddly, this seems like an inescapable conclusion. We are not guaranteed a better government with Democrats in office, but at least many of them believe that government has a legitimate role to play and that competence and honest efforts at wise policy can have positive consequences. I am a pessimist and have often noted that our best policy intentions have bad and unforeseen consequences. But if one doesn't believe in the enterprise from the outset the negative results are a foregone conclusion.