We are threatened with a much worse bill than this because republicans quit the party in 2006.
At the moment, the republicans are at least SAYING they are fighting, and at the moment this bill is at least a BETTER bill than the one last year, which may not be saying much but it suggests republicans might be the only thing standing in the way of much worse.
Quitting the party is something you can certainly do, but it won’t make things any better regarding immigration.
The Republican Party has done everything in making itself irrelevant. If I am going to get behind a small movement based on beliefs, I'll put my efforts into something that agrees with my beliefs more than the soon to be obscure Republican Party.