As cover-ups go, this is a big one, but in this case, the operation was in fact far worse than the cover-up.
You know that, and I know that, but from the standpoint of trying to get Democrats to cooperate, it's probably better to play up the significance of the cover-up. After all, it's likely that the vast majority of the Democrats had no involvement with the original crime, but will be unable to avoid involvement with the cover-up if they continue to obstruct the investigation. Further, from a damage-control perspective, if it seems that the cover-up is going to grow to be worse than the original crime, then it may be worth abandoning it so it doesn't grow so much, even if doing so would let the original ("minor") crime come to light.
Yes, the actual act in this case makes the entire Watergate scandal, coverup and all, look like flatulence in a hurricane. This compares to Watergate as the Mississippi river compares to the little creek I used to jump across when I was sixteen. Those who say things like, “As bad as Watergate” are only showing their total ignorance.