I disagree with your premise. I question that the GOP is really made of up hard-liner conservatives. You claim the Democrats are functioning as the party that just opposes their opposite, but I think that's the GOP. I'll give the Democrats this credit--I think they believe what they support. Right now, a lot of what the GOP has advocated is just done to oppose the Democrats.
The party out of power is always going to oppose the administration. That's just how it works. But the Dems were really crappy at opposing Bush, partly because we used to have so many Blue Dogs who represented Bush districts. Although some swingback is possible, I believe the sun has set on that era permanently. The next GOP prez is going to have more trouble than Bush did with the Dems.
All that said, the GOP is much more of a hard-line party. A lot of that is that there are a hell of a lot more self-identified conservatives than there are liberals in America. And part of it is that the GOP is full of authority-loving followers, while Dems have much of a wishy washy let's hold hands, split the difference, and sing Kumbaya culture. Alan Grayson aside, the Dems don't have many warriors these days. It's a shame because we used to. Bernie Sanders is as close as you get to a liberal warrior, and he's not even officially a Democrat. The GOP is chock full of warriors.