On the other hand, I checked how they fared in recent regional elections where they ran, and that does indicate a ceiling:

Baden-Württenberg: 2001-20,528, 2006-26,759
Rhineland-Palatinate: 2001-5,379, 2006-4,973
Mecklenburg-Vorpommern: 2002-1,312, 2006-1,957
Bremen: 2003-1,009, 2007-960
Hessen: 2003-6,674, 2008-did not run
Lower Saxony: 2003-7,819, 2008-5,851
Hamburg: 2004-1,571, 2008-did not run

_{*Lunatic*, n.
One whose delusions are out of fashion.}