Let's think about the modern history of "the scandal," and how such episodes emerge.
The modern saga all starts with Nixon. Obviously there have been scandals throughout political history, and in the immediate pre-Nixon era you had Bobby Baker, Billy Sol Estes and Walter Jenkins with LBJ; Sherman Adams under Eisenhower; and such different political dramas as the Army-McCarthy hearings in the early Eisenhower era and the Bay of Pigs aftermath under JFK. But Nixon marked the beginning of the modern scandal era. That is because the phenomena of the televised high-stakes public inquiries (as with the Watergate hearings and impeachment preparations) really dates to then, as does modern press-consciousness of how coverage of a big, exciting scandal looks and feels.
I. The main stops along the way:
Nixon: Watergate -> hearings -> dramatic revelations -> Supreme Court hearing -> impeachment -> resignation.
Also under Nixon: Spiro Agnew taking a brown paper bag full of bribery money, in his Vice Presidential office, and having to resign, something barely remembered now.
Nixon era: Teddy Kennedy and Chappaquiddick.
Ford: None really.
Carter: No long-running ones, despite flaps about his budget director and his chief of staff. But his era marks a major change, since round-the-clock news coverage was just getting going then, and Ted Koppel's Nightline, originally known as America Held Hostage, pioneered what we now think of as scandal-style coverage, of the American captives in Teheran.
Reagan: The Iran-contra mess, complete with Fawn Hall and Oliver North.
GHW Bush: None, really, though the Clarence Thomas nomination got scandal-style coverage because of the charges against him and the dramatic hearings.
Clinton: First the phony scandals of Whitewater and Vince Foster. Then the real problems via Monica Lewinsky. Clinton era notable for the creation/revelation of something like a permanent-scandal mentality in politics and the press.
GW Bush: Few scandals in the technical sense. But the election, recount, and judicial overreach known as Bush v. Gore got scandal-style coverage. Then Abu Ghraib, waterboarding and torture, and the war as a whole.
Obama: In his first term, the phony scandals of birtherism and Shirley Sherrod. Now the three-"scandal" combo made of elements that have nothing whatsoever to do with one another and don't necessarily have anything to do with Obama himself but that nonetheless satisfy that phantom-limb craving for a good exciting scandal.
II. What a Scandal Takes, to Take Off
1. An underlying offense people can understand. Clinton and Monica meet this test. Also Nixon ordering wiretaps, or Agnew taking a bag of money. Iran-Contra was always sort of a struggle on this front -- for people to grasp what exactly the offense was. Today's IRS/Tea Party accusation meets the test (despite complexity of the underlying reality); Benghazi, less so.
2. Evidence of president's personal involvement. The Watergate tapes again lead the way here -- Nixon's own voice, cursing and swearing. Monica and Clinton -- whew. Obama "scandals" lacking here.
3. A formal hearing/ investigative structure that guarantees an ongoing daily drip-drip-drip coverage. When there is a schedule of witnesses for a hearing, an upcoming set of votes, or a sequence of new revelations, then the story can keep going for weeks, months, even years. Darrel Issa, listen up!
4. A press culture and DC culture that is now wired to swing into "scandal mode," and start writing stories and giving commentary reflecting that "narrative."
5. A structure of news coverage that keeps the scandal narrative going. This was probably at its strongest in the era of the weekly news magazines (Time, Newsweek, etc.) Then you would have: daily coverage in the papers; nightly coverage on TV news; weekly advancing of the narrative by news mags (and Sunday talk shows); analysis of "Administration in crisis" and "President under pressure"; and it would all keep going. Now, in a sense, the hourly / minute-by-minute cycle can make scandals "burn out" too fast.