The thing that upsets most of us on the Left about Harriet Miers isn't her conservatism, since she has no record and no one knows whether she really is a conservative or not. What bugs most of us is that she is another example of the eternal Bush cronyism.
Therefore, we shouldn't be attacking her (letting her on the court might be a good gamble for our side); we should be using her confirmation hearing to attack Bush cronyism.
How to do this? Secret strategy on the flip...
As a longtime inside player, (first as "Assistant to the President", then as "Deputy Chief of Staff" before becoming the White House counsel), Miers was likely in on most of the shady stuff that has been going on in the White House, from the days of the energy task force (remember that?) through the torture memos and lies about WMD, up through our current Rove-Libby treachery on Valerie Plame. Right now, we do not know which ones she was in on and which she wasn't (and Bush is now refusing to release documents, of course). But we should pull together whatever we can from past news reports citing Miers, and the Dems on the Judiciary Committee should query Miers about her involvement in each and every one of the Bush scandals. Who did she speak to? What memos did she write or review? What opinions did she offer on the legality of this or that dirty trick? Etc.
This will be politically good for us, of course, because it will be perhaps the first time that we can get a White House insider to testify under oath about all the crap that has been going on for the last five years. The more truth about Bush's dealings we can get into the open, the better for our side.
But slamming Miers on Bush will also be good for the republic, because it will demonstrate to Bush and all future presidents that there is a definite cost to nominating unqualified cronies. (Of course, this should have been obvious with Brownie, but his mistakes only cost lives--the Miers nomination will result in a political cost, which is what Bush cares about). If Miers is used as a crowbar to pry the lid off this White House, then it will be obvious to all presidents hereafter that nominating someone close to you will open you up to personal political attack--and if that is obvious, then future presidents might actually decide that they should be appointing people based on their qualifications, not their connections.