Although immigration reform will permanently remove the Republican party from control over Washington, many Republican congresscritters support immigration reform. There are various reasons, but it primarily comes down to self-interest. Most Republicans are from "safe" districts, so immigration reform won't--over the short term--threaten their jobs. In addition, they have the potential to reap significant financial incentives to do so from wealthy and powerful donors--especially agriculture and businesses concerned with reducing labor prices.
The media (both liberal and conservative) tends to focus on the unskilled workers coming north from Central America. However, there is also a push to continue to increase the number of H1B visas for "highly skilled" workers--programmers and engineers, primarily. Most of the opposition to expanding H1B visa programs comes from those who point out that the allegations of a shortage of highly trained workers is a lie.
However, there is another aspect to consider, which is that as more H1B workers are allowed into a particular field or business, they will work to protect their own, and drive out American workers. Case in point is InfoSys. Computer World reports on a discrimination suit filed against the Company by an American worker who was harassed and discriminated against by a predominantly Indian work force. From the article:
The passage of the Affordable Care Act brought with it a burst of IT spending and hiring. The District of Columbia, for instance, hired offshore outsourcing firm Infosys for $49.5 million to build its Healthcare Exchange.
The India-based Infosys brought in H-1B visa holders to work on the government project. And of the approximately 100 Infosys employees working on the healthcare project, only three were American, according to a civil lawsuit filed in federal court.
The IT professional making the claim, Layla Bolten, has a degree in computer science and has been in IT since 1996. An experienced tester, which is what she was hired for, Bolten often helped less-experienced staff.
But the lawsuit contends Bolten was harassed because she was not Indian and excluded from work conversations by supervisors who spoke Hindi. People with less experience were promoted over her, and she eventually quit.(H/t Instapundit)