It would be interesting to see Huckabee vs Romney as the two main factions of the GOP Primaries in 2008. I’d imagine Huckabee still loses to Romney and then the economic populist Huckabee supporters turn elsewhere?
It was kind of shaping up to be that until the Surge issue took over the primaries. McCain was the only candidate who whole heartedly supported it, and he ended up being proven right by spring of 2008, when voting was going apace. In mid 2007, there was a lot more skepticism over it and Ron Paul and Tom Tancredo briefly got a bit more support as a result.
If Bush decides on no surge, and goes the cut and run route in Iraq, I think that it might have ended up being a class war of sorts.
And this is something that many people forget, but Huckabee did well with two groups of voters, the very youngest, and the very oldest. The youngest voters in the GOP Primary (many of whom were heavily influenced by CCM and Christian Youth organizations, which were very big at the time) were enthusiastic about his religiously tinged messaging and somewhat radical ideas about taxation (he was in favor of the FairTax). The oldest voters liked his economic populism and focus on blue collar jobs, along with his almost militant commitment to social security and medicare (entitlement reform in the GOP in the 2000s was a lot more popular than it is now, mostly because the voting coalition was much more upscale and suburban), which other candidates did not share.
Romney, however, would have won the primary, most likely.
As for Huckabee's voters in that case, my guess is that most stay with the party (albeit with depressed turnout) but a portion would switch over if Hillary was the nominee in 2008 (she ran a pretty blue dog campaign that year).