Always been pretty unbelievable, yet entertaining. The early murders and the president laying down so easily were the most ridiculous to me.
I agree that it has never been extremely believable, but the Claire plot arch has gone from unrealistic to silly. I still like the show, but the Underwoods really have no business winning a presidential election.
Could Bill have won the 1996 election if Hillary was his running mate? I think the Underwood ticket winning would be about as believable as Gerald Ford winning in 1976 if Ford was less likable, more corrupt, and made his wife his running mate.