For some reason, real investigations make them very nervous. Via Axios:

Congressional Republicans are getting ready for hell. Axios has obtained a spreadsheet that’s circulated through Republican circles on and off Capitol Hill — including at least one leadership office — that meticulously previews the investigations Democrats will likely launch if they flip the House.

Why this matters: Publicly, House Republicans are putting on a brave face about the midterms. But privately, they are scrambling to prepare for the worst. This document, which catalogs requests Democrats have already made, is part of that effort.

[…] The bottom line: Thanks to their control of Congress, Republicans have blocked most of the Democrats’ investigative requests. But if the House flips, the GOP loses its power to stymie. Lawyers close to the White House tell me the Trump administration is nowhere near prepared for the investigatory onslaught that awaits them, and they consider it among the greatest threats to his presidency.