'Why do Republicans eat their own? Identity politics...'
1. Lack of Leadership & Unity
Reagan unified the party, and one of Reagan's Rules was for Republicans to never attack / talk bad about other Republicans, at least in the public eye - something the Democrats have mastered...for the most part. (Bernie has been violating that a little lately, resulting in Hillary 'snapping' at a Bernie supporter the other day.)
2. Ego / GREED / Lust For Power / Desire to MAINTAIN Power
Let's be honest: Millionaire, elitist self-appointed Oligarchist 'Rulers' - also known as our 'beloved' career politicians' who have thrown aside their duties of 'elected servants of the people' for their new self-given titles AS 'Rulers' - have expended a great many years and much effort to acquire their wealth, positions, and power. They have given themselves the power to be exempt from the law, have given themselves raises whenever they feel like it - despite their failures or inability to do anything OR how much the country suffers. They will do anything - ANYTHING - to keep / maintain all of that money, power, positions, and prestige. That includes attacking citizens from the floor of the House or Senate, using the IRS to attack political enemies, and even bashing their fellow Republicans who threaten all of this (as Trump does) - to include fellow members of Congress.
3. FEAR:
This goes hand-in-hand with pt #2. Trump scares the hell out of politicians on both sides, to be honest. When Trump says he does not owe anyone, that when he gets in office and sees a problem he will call upon the BEST, MOST QUALIFIED person/group to quickly resolve the problem what he is really doing is THREATENING the 'good ol' boy', 'political', special interest, Union, 'you-scratch-my-back-and-I-will-scratch-yours-and-we-will-all-get-rich' system that plagues our government but which has made elected officials who go to Washington filthy stinking rich...and corrupt. No one up there wants that to change, and Trump quite frankly frightens them.
When the (original) TEA Party rose up, at the height of it's power, it helped swing elections, helped the GOP take back the House. The GOP was ecstatic how this movement seemingly went after Democrats....until the TEA Party started turning its attention toward the GOP as well, beginning to hold the GOP leadership as equally accountable. Boehner and McConnell didn't like THAT, so they began a war with the TEA Party. In the last election McConnell even walked out on-stage at the convention holding a rifle, began talking about Conservative Principles, talked about the GOP 'Big Tent', then declared there was NO ROOM under that 'tent' for TEA Party Conservatives. The truth is he recognized them as a threat to POLITICIANS, not just Democrats.
Obama recognized them as a threat to him and fellow Democrats, which is why the 'rogue members' of the IRS began quietly targeting 'Patriots', 'TEA Party', and other such groups. There was nothing quiet about Boehner's and McConnell's approach - they declared war on this new upstart group that threatened their grasp on Washington and the power that came with it.
Trump is a manifestation of the people's frustration and anger, a 'Frankenstein' created by Boehner, McConnell, and all the other WE-GOP who stopped representing and speaking for the people and went 'all-in' for themselves. So they are, again, openly trying to destroy him - publicly - and in doing so 'squashing this 'radical' notion that Republicans have come to believe, that THEY control the party and have a say in how the government is run. We can't have that now, can we! The people must be put back into their place.'
Just my humble opinion...