The Democratic party is still shopping for a spine.
Again, I will apply true rocket science to the issue.
Everybody knows who fired the U.S. Attorneys: Rove and Bush. Everybody knows why they were fired: Because they wouldn't interfere in elections or would prosecute Republicans. Everybody knows that the U.S. Attorneys who weren't fired did interfere with elections and/or wouldn't prosecute Republicans. This is common knowledge.
The only thing left to do is either get these guys on C-Span, under oath, or allow Bush to provoke a Constitutional crisis by refusing to allow them to testify. Either way the point is to build a case for impeachment.