Is it the fight on the right that does them in for a generation? Or the demographics change? Or will it be a combination of both? I'm leaning towards the guess of both.
Look... their economics on the whole of the right is an abject failure, and is being pulled into different directions of failure between the Austrian School weenies, Neoclassical dummies and (funniest of all) the Ayn Rand acolytes. Their hatred towards people of color and immigrants is repulsive to people of color and immigrants. And their base is dying off from old age.
The fight on the right has been brewing for about 3 decades now. It began when the religious right came up with the "Moral Majority" and that morphed into the "Term Limiters" who turned into the "Crotch Watchers". They assumed that they had "won" when they put "their man in the WH" only that turned out to be nothing but pandering window dressing. They changed their name to "Main Street America" for W's 2nd term and were falling apart in 2007. But Ron Paul managed to coalesce the libertarians into the original Tea Party and Sarah Palin energized the "Birthers".
Then we had the 2008 economic collapse and the hijacking of the Tea Party to generate support for the anti-ACA crusade. The 2010 elections were a huge boost to the extreme right because there was considerable justifiable anger over the state of the job market. This was exploited and finally resulted in a sizable number of extremist candidates winning seats in both the House and the Senate.
2012 turned out to be a setback because the gains that the extreme right expected to make never materialized. Quite the opposite and that is why we are now approaching the semi finals for the fight on the right. 2014 is the last real chance for what is now the Tea Party to prove that it can win elections for the GOP. If they take a majority in the Senate then they will be able to dictate who will be the candidate in 2016. But if they fail then the establishment GOP will try to retake control and run a moderate candidate instead.
The fight on the right will occur when the establishment tries to reassert control over the Republican Party. The establishment knows that there is a demographic shift and that an "all-white" electorate is no longer a viable power base. The need to appeal to a broader base means embracing the things that have meaning for those parts of the electorate.
So we are at the turning point heading into November. It will be the gauge for the relative strength of each side. This election will hinge on the Senate races and while it is true that the Dems will probably lose seats the question that needs to be answered is how many?
In 2012 it was the extreme right that cost the GOP Senate seats that they could have won with establishment candidates. 2014 is shaping up to be a similar showdown. If the Dems hold 50 seats after the election the Tea Party will probably take the blame in my opinion. If the Dems are reduced to 49 or less the Tea Party will claim a victory and demand that they get to pick the 2016 candidate.
Going to be an interesting election to watch in my opinion.