WMDs in Iraq - not a Party cover up. All intelligence agencies thought Iraq had WMDs and it was only when an invations was imminent that some doubt krept in. The UN though he had WMDs. I don't think it was a cover up as much as an excuse to take Saddam out after 9/11.
1980s Reagan mental competence, Iran/Contra - Reagan wasn't incompetent and the CIA and his staff engineered Iran/Contra. It may have been a cover up, to some extent, but it was quickly exposed and dealt with. Nowhere comparable to the Democrat Party today.
Nixon Watergate coverup. Vietnam--Cambodia/Laos bombing campaign. - Watergate was not a cover up by Republicans and, in fact, Republicans called for Nixon to resign or be impeached. Vietnam-Cambodia was a Nixon cover up, but again, not one supported by the Republican Party.
The media does a much better job exposing any Republican incidents. The Party doesn't rally around people like George Soros like Democrats to Bob Menendez.