Guys, remember that 1933-1981 was a period of almost continious Democratic dominance in congressional elections (1933-1995 if we discount six years of Republican control over the Senate, as Democrats retained the House).
I think the answer to OP's question is pretty much this, as well as the fact that Nixon and Ford were actually pretty good candidates. Nixon was a fairly popular VP running to continue the legacy of one of the most well-liked presidents of the 20th century, and he (along with FDR) is one of just two people to be on four separate winning presidential tickets. Ford, meanwhile, somehow nearly won an election that took place just two years after his Republican predecessor resigned (there's some circular logic there, but I don't think he should just be written off as a weak candidate).
