Get beyond your impressions of and feelings about the Bush-Cheney regime. Even if you’re a supporter. Look to the essential components of the American Republic.
Congress is marginally in control of Democrats. If their control expands in the next election, they might develop the courage and the wherewithal to assume the powers delegated to them by the Constitution. [NOTE: Congress is not entitled to exercise power. It is obligated to exercise power.]
The Presidency is not entitled to as much power as has been usurped by the current Administration. That arrogance will only increase if a Republican is elected; whether he chooses it or no, a Republican President will be swept along by inertial power of the system. A Democrat may want that much power, but will have to start from the beginning to acquire it.
Right now the Presidency may not be as important for itself as for two other institutions. They are the only critical institutions in this country not subject to periodic review by voters or clients or stockholders: the Supreme Court, and the military.
Conservatives hold a tenuous majority on the Supreme Court. The next two justices to retire both are liberals. If their replacements are named by a conservative Republican President (there being no other sort in the running), conservatives will dominate the Supreme Court for decades to come, and no way to stop them. The only chance for balance is for a Democrat — a centrist will suffice — to name the next two justices.
Our military forces have always remained under civilian control. When as now that civilian control is in the hands of monomaniacs and fools, none of them having significant military experience of his own, the military is directed to unprincipled and often illegitimate action. For principled military officers, this presents a dilemma: resign, find compromise solutions, or abandon principle. For unprincipled offices, there is no dilemma.
[NOTE: The vast majority of our military leaders have reached the top through experience and training and demonstrated capability, qualifications not imposed on politicians.]
Continued control of the military by an expansionist regime will increase the proportion of unprincipled officers. The entire military service will, over time, develop an enthusiasm for international adventuring. Continued reliance on mercenary forces, who are by definition less disciplined and less principled than regular troops, will exacerbate that problem.
In these regards, the worst Democratic candidate is absolutely preferable to the best Republican candidate. The only way round is to elect a Democrat, no matter who.