Although it is way to early to predict doom for the Republican Party, however, just as a snowball gets bigger when it is rolled down a hill, enough is now known that the Presidential election prospects look very bleak for the Republicans. Donald Trump is a Mountebank and will lose to Hillary Clinton with near certainty. Perhaps not as much as Barry Goldwater lost to President Johnson in 1964 or Senator McGovern lost to President Nixon in 1972, but Clinton’s victory will be at least as big as then Senator Obama’s victory over Senator McCain in 2008. If the Republican Convention in July maneuvers to give the nomination to Senator Ted Cruz (clearly the most unpopular member of Congress), Cruz will lose almost as badly as Trump and Trump’s die-hard supporters will be mad as hornets.
Assuming that the Senate flips back to the Democrats (unless there is a third Party Conservative candidate to provide cover for some of the vulnerable Republicans), how would a lopsided victory by Hillary Clinton affect the House Republicans? Suppose that Clinton subtracts 7% from every Republican’s two Party percentage from 2014 (the horizontal line in the figure below — my thanks to Gary Jacobson for suggesting this number), then the Republican Caucus would come in around 220 members. This would be enough to retain “control” but the Republican Party will be badly split by either the nomination of Trump or Cruz and it is safe to say that the far, far, right will be even less likely to be cooperative in passing necessary appropriations bills. Suppose this “suicide” caucus is to the right of 0.65 using CS DW-NOMINATE Scores and above the 57% line. This will leave 41 members to gum up the works in 2017.
To get anything done, President Hillary Clinton will have to negotiate cross-party deals in the House. This, of course, will further infuriate the “suicide” caucus and it could lead to a permanent division within the Republican Party.
The last major Party to break up were the Whigs from 1851-1854. We may be living through an equally historic period. Time will tell.