WASHINGTON.- In an effort to attract tourists to the District of Columbia, local officials have launched a $500,000 campaign revolving around the exhibition "Jacqueline Kennedy: The White House Years," on view at the Corcoran Gallery of Art since April 6. The marketing promotion has involved nearly 60 D.C. organizations and targets markets in Pittsburgh, Philadelphia and Atlanta, among others. The show opened at the Metropolitan Museum last summer and subsequently appeared at the Kennedy Library in Boston.