From "Is the Presidency Failing?," an article by Donald Horowitz in the summer issue of The Public Interest:

Of the institutions that define the incapacity and the scandals that make for presidential failure, perhaps the foremost is Congress. To be sure, the institutional interest of Congress in presidential failure is tempered by the interest of congressional members of the president's own party in reelection; but between elections Congress can benefit from exposing executive misfeasance. At such times, the branch asserted to be mired in special interests is able to support rectitude and clothe itself in the law.

It is not really possible to have a scandal or even a single profound policy failure in Congress. Congress is not a coherent, corporate body; it cannot be accountable as one. The major congressional scandals of the postwar period, most of them involving sexual or payroll peccadilloes, did not rub off on the institution. The presidency, by contrast, remains a corporate entity, on which responsibility can ultimately be fixed. In fact, its corporate character is a major reason for the expansion of the White House staff, to accomplish something close to what the president wants to accomplish. The White House is a creature of its leader's style of advice taking and decision making, as virtually every study of the presidency attests. The White House is also a creature of its leader's personal failings, whether they be Carter's excessive concern with detail or Reagan's neglect of it, Johnson's humiliation of subordinates or Nixon's encouragement of their fortress mentality. In Congress, the styles and flaws of members are mainly interpreted as idiosyncracies of their office; they rarely extend even to the committees they chair. Misfeasance disgraces only individuals.