The problem with the Republican Party is that it is composed of various groups—small government fiscal conservatives, social conservatives, national security hawks—whose interests don't always align.
Usually the GOP can find a candidate, like George W. Bush, who satisfies all three groups. This time around, though, they're not so lucky. The candidates who "check all the boxes" are either faking it (Romney) or are only kinda interested in running (Thompson), and those who don't are getting beaten up by the portions of the party they leave dissatisfied.
Take this ad, for example. The fiscally conservative, tax-hating Club for Growth is hammering Huckabee for his moderate economic record.
The bad thing about Huckabee's tax hikes mentioned here is that they weren't progressive (except possibly the income tax "surcharge"); they hit the middle and working classes as hard or harder than they hit the upper class. That means that if Huckabee survives these attacks at gets the GOP nod, he'll probably be attacked for this stuff by the Democrats, too.