It isn't immigration that's marginalising the Republican party. It is 8 years of disinformation and conspiracy theories that Obama is a Muslim / atheist, Kenyan / Indonesian, Communist / Fascist, dictator / wimp who is both lazy and works 24/7 to destroy America - which has resulted in a rabid base that cheers a mentally unstable, unfit con artist of a nominee when he trashes the military and praises a former KGB agent turned dictator.
The base doesn't give a damn about fiscal discipline, individual liberty, national security, international allies, morality or even decency anymore. All you have is white iden y politics and a burning hatred for Obama, Hillary and the Democratic party. Trump's candidacy is the equivalent of a long overdue enema for the GOP. However, this will only end well if he loses in a landslide and the GOP overhauls itself. Otherwise the nativist / bigoted fringe takes center stage, and down the rabbit hole we go.