The Republican party as it's constituted today doesn't actually stand for anything in particular. It stands against liberalism, but that's not exactly coherent.
What the Republican party has become now is the party that's against Barack Obama; everything Barack Obama stands for. If Republicans had been right that Obama was a crazy leftist, then by opposing him at every turn - which is what they've done - they would have made their party a moderate, centrist, politically popular party.
Instead, Obama was a moderate; a practical centrist. And therefore, by opposing him at every turn; by holding the nation's credit rating hostage in the debt ceiling talks; by coming out against the Iran deal when they had no alternative but war; by becoming the party that's against not just illegal but LEGAL immigration, they have pushed their party to the extreme. And that is what has left them in a situation where Donald Trump now represents their voters.
As far as I'm concerned, anyone who uses the term "liberal" to describe Obama is as disregardably ignorant as those who call him a Muslim or a Kenyan. He's been the very definition of a moderate (unless you're so far to the right that the center seems like the left).
While, as a moderate/centrist myself, I'm unusually open to differing viewpoints, I've decided that I no longer consider this an arguable point.
Update: see Saletan's reasoning at greater length here