From the longer English Wikipedia page [1] - there is more bibliographical detail on the German Wikipedia page [2].
Jakob Maria Mierscheid MdB (born 1 March 1933) has been a fictitious politician in the German Bundestag since 11 December 1979. He was then the alleged deputy chairman of the Mittelstandsausschuss (Committee for Small and Medium Sized Businesses) of the Bundestag in 1981 and 1982. According to his official biography, he was born in Morbach/Hunsrück, a very rural constituency in Rhineland-Palatinate. He is Catholic and a member of the Social Democratic Party of Germany.
The Mierscheid hoax was said to have been originally introduced as early as in the 1920s by Weimar Social Democrats to avoid paying restaurant bills. He is now a widely known curiosity within the Bundestag and uses Twitter as means of communication.
In 1983 the party magazine Vorwärts published the still valid (as of 2002[update]) Mierscheid Law and claimed Mierscheid as the author, demonstrating a correlation between federal election results and West German industrial production.
The Bundestag official web site carries an ostensibly serious 'biography' and a photograph purporting to depict Meirschied.[In previous versions of the photograph his fashion sense seemed very antiquated and his eyeglasses were added later. The current (2010) image shows a balding man sitting in a chair, facing away from the camera, in the middle of the empty Hall of Representatives. The site lists 615 current names although the actual membership of the Bundestag is only 614. Mierscheid has his own stationery and e-mail address and issues press releases now and then. The picture of Mierscheid at the Bundestag is based on the RTL Samstag Nacht character Karl Ranseier.
The hoax is paralleled in Germany in a number of other areas, for example Friedrich Nagelmann is a known (fictional) lawyer and Edmund F. Dräcker is a known (fictional) diplomat. Mierscheid, Nagelmann and Dräcker each have a long list of publications which have sometimes really been published in otherwise reputable media (science journals, parliament press) - non-native readers might be fooled here (compare April Fools Day).
They are used as well as test persons in questionnaires, to allow to distinguish between the real and guessed popularity of certain persons.
The Deutsche Bundestag page is [3].