Someone's going to blast me as a "partisan" for saying this but here's my take on why the Republican congress fell so far away from their principals.
It's the Democrats fault.
Before some of you go off the deep end listen to my reasoning. The Republicans rose to power because of a deeply flawed Democrat party that believed it was the only party "entitled" to lead the Congress and was driving the country to ruin. The Republicans developed an agenda based on substantive changes needed to reverse the downward trends in the economy, military, social issues and the rise of government intrusion in everyone's lives.
OK, so the Republicans gain power based on these substantive arguments. You can argue that you disagree with some of their positions but can't argue that the issues were not substantive.
So, what happens then? The Democrat party basically went institutionally insane and, as a party, began attacking absolutely everything about Bush. He was "illigitimate", he planned the war back in Texas, he did it to make his Daddy happy, he's worse than Saddam, he's the world's worst terrorist, he knew about (or planned 9/11), etc., etc., etc. The Dems have yet to develop a substantive agenda or offer anything other than that they aren't Bush/Republican. Totally insane.
So, now we get to why the Republican party lost it's way. How many people here like football? How many have seen powerful teams lose to losers because they didn't see them as true competition and couldn't conceive of losing to them? We'll, that's what I think happened here, the Republicans became fat and lazy and began to feel entitled to lead. By almost every measure the Democrat party didn't represent a realistic threat and the Republicans' felt too safe and began to feel they couldn't be defeated because the American public couldn't possibly elect a bunch of losers over them. In other words they lost their way because there was no legitimate threat so they didn't have to toe the line and deliver. What they forgot is simply the possibility that the American public that elected them would not stand for them to turn into the Democrat party of old, full of themselves and believing they're entitled just because they have an (R) listed behind their names. So we ended up where we are now, an incompetent and agendaless party in control, and only in control because they aren't Republicans, not because of anything they stand for.
That's why I've always said that, while I don't like the Democrat party or it's various agendas, an insane Democrat party is worse than a strong one. Being insane makes it easy to dismiss them, being strong would have forced the Republican's to stick to their agenda.