In roughly three months President Obama is going to destroy the Republican Party as a national institution (it will be reduced to its Southern and Western rump). It all comes down to the trap he has laid for the Republicans on immigration. When the House refused to even take a vote on the Senate bill last year, it meant that no immigration reform would ever come out of Congress. This has freed Obama to take executive action, but he has had to wait till after the midterms before he unleashes the sh*tstorm about to come down.
When he did DACA via executive action it was at that time in response to Republican refusal to vote on the DREAM act, which freed him up to deal with the problem directly, and despite some carping on talk radio, the Republicans swallowed it. Even now their candidates run in terror when confronted by DREAMers. So around January or so, he is going to do the same thing for the rest of the undocumented. If you are here, have no criminal record, and want to work, he will stop deportations and allow work permits. This de facto legalization (but not amnesty, as they will not have green cards much less a path to citizenship) will be wildly popular with liberals and Latinos. As the economy is now generating strong job growth the average American will not see this as an economic threat. But it will drive the Republicans insane. They will have no choice but to completely oppose this at the level of every elected official, and it will include everyone running in the 2016 primary.
This will be the death knell for the Republicans. Because Obama's executive action can be overturned by the next President, the 2016 election will pit a Democrat in favor of the policy continuing, and a Republican aggressively committed to reversing it. The Republican wipeout with Latinos will be epic, and will drag down every blue state Republican Senator andd many Congressmen with it. This will be a bullet they can't dodge, because any Republican that refuses to condemn and oppose Obama will be primaried, and the 2016 Presidential nominee will have to explicily vow to undo Obama's act in order to win the nomination.
The 2014 midterms wont really alter this dynamic. If the Dems hold the Senate, Reid will bring the Senate bill back to the floor and either McConnell votes for it or he filibusters but regardless, Boehner can't allow it to see the light of day. If the Dems lose the Senate, Obama just has to say put up or shut up, if you don't like my executive action, then the R's just need to send me a comprehensive immigration bill, which they can't possibly put together with the Cruz/King clown show in charge of immigration. One more Presidential election that puts 70% or more Latino voters on the Dem side will consign the Republicans to permanent minority status, until an utterly changed Republican Party can take shape, but it may be it just limps on for another few decades representing the reactionary third of America but never holding the White House again, and losing the Supreme Court in the next decade as Scalia and Kennedy pass on.