So maybe with Kerry winning in 2004 we see sizable Republican gains in 2006 of 20ish seats in the house, and probably a wash in the senate. Turning a dem midterm into a R midterm radically changes things, but I can't imagine it being enough for Santorum to win his race. Rs probably lose PA, but they might take NJ and have an outside shot at MD (in a D+8 wave both were 9-10 point wins for dems so its not unlikely that in an R+4-5 (or maybe more) environment that Republicans can win those seats) which would put the senate at a 1 seat gain for Rs. I would also say that I think this scenario prevents a lot of the losses Rs saw with hispanic voter between 2004 and 2008, it wouldn't surprise me ITTL if the hispanic vote stays around 40-45% for Rs although it would probably be even higher in Republican landslide years like 2008 should be.
So coming into 2008 Republicans have 56 senate seats and somewhere around 250 in the house. Lets say Huckabee does win the nomination. Its hard to predict the presidential election here, because Huckabee is very likely going to win significantly, but also his evangelical christian conservatism is definitely going to hurt him in some states Republicans could otherwise win in a 2008 R landslide scenario. Do voters in states like NH, NM, OR, NJ, and MI just swallow the downside of Huckabee and vote for him anyway because they're so fed up with Kerry, do they decide Huckabee is just too much and vote for Kerry, or do they just stay home? Its honestly hard to say, and I think it depends on how Huckabee campaigns and how he presents himself.
As for the senate elections in 2008, I think Republicans hold AK, CO, MN, NH, NC, and OR. They might also hold NM if Domenici runs again. This means only VA goes democrat. In addition, I think Republicans could potentially flip several seats (some of which weren't even competitive OTL) such as AR, LA, and maybe even IA. lets say Rs win 2/3 of those potential flips, that would put them at 57 senate seats if im doing my math right. Not enough to break a filibuster on their own, but likely enough to pass most of whatever they want to considering that a lot of social issues still had conservative democrats at the time (on things like gay marriage and flag burning and that type of stuff). Potentially there could be another 5-10 seat gain in the house, but Republicans would surely be almost maxed out there.