At an emotional level, I agree with Jack Hitt: Democrats shouldn't even show up for the vote to confirm Miers. This country is fast becoming a banana republic and the best thing the party can do is to let voters know who holds the reins in Congress. More "seriously," though, I don't really know.
Obviously, the key consideration is this: "If Miers is defeated, would her replacement be better or worse?" With that in mind... Whatever else one can say about her, say this: Miers is an administration hack of the first order, utterly subservient to the Bush family. She will almost certainly rule Bush's way in a number of upcoming Supreme Court cases—Hamdan v. Rumsfeld, Padilla v. Rumsfeld, challenging the authority of the president to detain and torture whoever he wants without Congressional oversight. This, to me, is the most important issue on the Court's docket in the near future, along with abortion. The Senate recently passed a bill regulating treatment of detainees in U.S. custody. How long will that bill last? The White House OLC, under Alberto Gonzales, has argued that "any effort by Congress to regulate the interrogation of battlefield combatants would violate the Constitution's sole vesting of the Commander-in-Chief authority to the President." As a legal matter, I think this is flat wrong. As policy, it's disastrous. But would Miers endorse this view, or something like it? Another, more "principled" conservative might put limits on Bush, although that's a gamble: Antonin Scalia has argued, in Hamdi v. Rumsfeld, that American citizens have a right to challenge their imprisonment, but "enemy combatants" abroad do not; Clarence Thomas, meanwhile, basically believes the executive branch can do whatever it wants. Miers will almost surely take Thomas' view; if she was defeated or her nomination withdrawn, her replacement might take Scalia's slightly-less-bad view, which would be better than nothing. But maybe not. On matters concerning things other than the executive branch, Miers is likely to vote no more conservatively than anyone else Bush might nominate. From what we've seen, she might cast a few surprise liberal votes on social issues, especially when it comes to criminal justice, while taking a more consistently pro-business line than a "principled" originalist might do. (Of course, there are very few principled originalists anywhere—see "14th Amendment, affirmative action and"—so this doesn't really matter.) The danger with replacing Miers with a more qualified and "knowledgeable" Justice, one who has a firmer grasp of constitutional issues, like Michael McConnnell, is that a persuasive replacement could potentially convince the centrists on the Court—Breyer, Souter, Kennedy—to swing further to the right. Dahlia Lithwick has suggested that Scalia's antagonistic temperament has alienated many of his colleagues, and Marisa Katz has noted that Rehnquist was unable to convince the liberal justices on his court to sign onto his opinions until he became more likable and less harsh. I already worry that John G. Roberts will be more effective than Rehnquist at this, and we don't need another like him. Politically, if Miers' nomination was sunk, that might harm the Bush administration, but based on history, Bush's ability to ram stuff through Congress seems unrelated to the fact of individual victories or defeats. The Bernie Kerik fiasco didn't hurt the White house, and neither would this. The administration's frequent bumbling of late seems mostly due to the fact that Karl Rove is focusing on staying out of jail. One good thing that could come out of a Miers confirmation would be that evangelical turnout in the 2006 midterms might be depressed; but on the other hand, William Galston and Elaine Karmack have recently observed that evangelical turnout has been mostly constant since 1988, so this seems pretty unlikely. Prediction: The current conservative infighting over Miers will have precisely zero effect on anything substantive, nor will it harm the Republican Party in any way. Read Stanley Coser. Six months from now, they'll have forgotten all about their little grumbling.