I'd say that if it has already happened, since there are pretty much no moderate Republicans left that are serving on the national stage (other than Snowe, Collins, Scott Brown and Lugar in the Senate, I can't think of anyone).
I'd say that the short-term effect is that the emergence of a sizeable faction of Blue Dogs that used to be Republicans that would give the Democratic Party troubles in Congress and make things easier for the Republicans.
In the long run, however, demographics and the two-party system catch up with the GOP. The hardcore conservative base, which is primarily white, Christian and somewhat older, dies off and moderate-conservatives retake the party. The Democrats lose their monopoly on Hispanic voters as the Hispanics become more and more comfortable voting for moderate (i.e.-not "round up all illegals and build a 20-foot high wall across the Rio Grande") Republicans who realize that the party can't afford to keep alienating Hispanics.
The Democrats, for their part, remain where they are politically, but with a large portion of un-repatriated ex-GOP Blue Dogs for decades that moderate the party in areas where they are dominant. The liberal backlash to the conservatism of the old GOP means a powerful left wing of the party (relatively speaking, of course) will clash with the unreconstructed GOP Blue Dogs for control and could lead to multiple defections.
And, as a happy note, the hardcore conservatives are reduced to a faction in the GOP instead of the dominant majority, which some of the more socially conservative members especially being shunned nationally by the party in light of more increased public indifference on social issues (gay marriage legalized, marijuana decriminalized, among other issues).