Breaking it down.

The recent inter-Republican debates over immigration lead to discussions of a split in the GOP over this issue.

Okay... let's assume there is a split, and the hard-liners on immigration decide to break off. How might this affect the relative dominance of the parties? The answer could be interesting, if one looks at the conservative-moderate-liberal split in the US. I'm going to use CNN's exit polling data from the 2004 election, and a rough explanation from Rasmussen Reports on its HillaryMeter.

The CNN breakdown is as follows: Conservative 34%, Moderate 45%, Liberal 21%.
The Rasmussen breakdown was: Conservative 35%, Liberal 18%, and I will assume 47% moderate.
The 2004 Presidential race results were as follows: Bush won 50.7%, Kerry 48.3%, and 1% for minor parties. The 1% for the minor parties split about evenly between the left (Nader and Green Party candidate David Cobb) and the right (Libertarian and Constitution Party candidates, plus other minor candidates). I'll take a half-percent off each candidate.

Let's assume that Bush didn't pick up a significant number of liberal votes, and Kerry didn't get much from the right, either. So, Bush started with a base of 33.5 or 34.5%, and Kerry started with 17.5 or 20.5%, depending on which breakdown we use. This means that the moderates went for Kerry by 27.8-17.2 if one accepts the CNN numbers (based on Gallup). The Rasmussen numbers indicate a 31.8-15.2 split for Kerry (and remember that Rasmussen was largely on target in 2004), or the GOP loses two out of three moderate votes.

How might a GOP shift towards a posture akin to Kadima (or Shinui) change things? If the Rasmussen numbers are correct, should the GOP's push outrage enough hard-line conservatives into staying home or visibly bolting (as some are inclined to do). But this is going to pass to a degree. Tempers fade over time. But even if the GOP loses 3% of the hard-core conservatives, it does not follow that they will necessarily lose elections as a result.

You see, many of the moderates might otherwise vote GOP if it were not for hard-liners, particularly when said hardliners have used a lot of extremely harsh rhetoric, like labeling those who disagree with them on immigration as Quislings, or prove themselves to be willing to tolerate pretty slimy stuff. They might not support illegal immigration, but they disapprove of things like bigotry as well, or do not want to see a mindless enforcement of the letter of the law cloaked behind the phrase "rule of law". They have many of the same objectives as conservatives, but might be worried about the methods.

In other words, taking Rasmussen's two-to-one rule, the GOP stands to make more gains among the moderates. That 31.8 to 15.2 margin for Kerry in 2004 could shift to 25.8 to 21.2. That would mean that the Democrats and far-left would get 42.3%, the far left gets .5%, a far right party would get 3.5%, and the GOP would pick up a larger percentage, or 53.7% of the vote. That turns into a landslide for the Republicans.

If the Senate stands firm, they could help theRepublicans down the road.