Prescott Bush's NAZI connections are well documented but I suspect the reason Democrats leave this subject alone has an awful lot to do with the fact that old Prescott had plenty of company, both Democratic and Republican, with respect to his NAZI dealings. Many of America's elite from the 1930s found much in National Socialism to support; one of the more disreputable elements being eugenics (this was an even more popular idea in Britain). Perhaps even more fascinating is the whole hearted financial support Wall Street banks gave to Hitler AND Stalin (the money Hitler used to rebuild Germany didn't grow on trees and where do you think Stalin got the money to fund his Five Year Plans?). Prescott clearly supported the NAZIs (board member of AG Farben I believe until Speer nationalized the company in 1943) but Democratic Party icon Averill Harriman along with his company Brown Brothers Harriman was heavily invested in Stalin. As is typical in American politics both of the major parties have filthy hands and if they sling too much mud at each other there is always fall back. This is something the GOP seems to occasionally ignore. Still, your point is an interesting one since historically, Republicans seem to be the party with the most dirt under their collective carpet but they are also the first and often the only party slinging mud.