Worse still, in a country whose history means there is no tolerance in mainstream politics of neo-Nazi sympathies or anti-Semitism, they have suffered the embarrassment of having two members outed as former members of the far-right National Democratic Party (NPD).
The NPD also taps into disillusionment with mainstream politics but espouses xenophobic views and disagrees with the German constitution.
It is also suspected by police of links to a small far-right cell discovered late last year that carried out a decade-long murder campaign against foreigners.
The two members with an NPD past resigned late last year, but the Pirate's federal chairman Sebastian Nerz said there were "almost certainly a few more Pirates who used to be NPD".
Reacting to Delius's comment, Nerz told the top-selling Bild daily: "Everyone should think properly about what he says, about the historical analogies he draws and what effect they may have."
In a commentary, Bild said parties without an understanding of history had no place in parliament.
A candidate for the Pirates in a regional election in May in Schleswig-Holstein told Delius on Twitter: "You have not made my life any easier just now."
Other parties, which have watched the Pirates' rise with unease, jumped on Delius's gaffe.
Claudia Roth, leader of the Greens, who have suffered particularly from the Pirates' success, called the remarks an "outrageous transgression" that could not be excused by the party's lack of experience.
Senior Social Democrat MP Thomas Oppermann said the "tasteless" comparison showed the Pirates had yet to clarify their view of far-right militancy.
One Pirate candidate recently criticised Israel in a YouTube clip. Bild quoted the blog of another, Bodo Thiesen, as suggesting that Germany had acted in self-defence in 1939 in attacking Poland because the Poles had ordered a general mobilisation.