it becomes progressively less believable (as in, it gets really hard to maintain the standard suspension of disbelief)

One of the primary issues with it is that it's based off a plot that fits the British political system- and so things just seem weird. Urquhart, the British version, eventually becomes cevzr zvavfgre, juvpu vf n uhtr cebzbgvba. Underwood eventually becomes ivpr cerfvqrag, juvpu npghnyyl frrzf yvxr n qrzbgvba ng uvf ntr.