It's deeper than that, partisan politics has convinced a lot of gullible people to make their shitty party part of their identity. So any attack on the party is an attack on who they are.
Which is utterly idiotic because if those people turned off their kneejerk emotional responses for five minutes and looked long and hard at their party they'd probably realise that it barely represents them at all. And they probably despise a lot of these people as much as their opponents do, but can't admit it because that would show weakness in front of The Enemy. We can't attack our own, The Tribe is under attack from Outsiders!
Of course, that's exactly what political parties want. Because they know that they are uncharismatic garbage that poorly serves the interests of their voters. So judged on their own merits, only a fraction of their voters would still vote for them. It's too hard to make good policy, even if you made a good one it's really easy for the other side to spin it as terrible. So why bother? Instead make it about identity over substance. It's a little flag to wave and nothing more. Constantly divert attention away from who you are and onto the opposite team.
It's very hard to make yourself liked. But really easy to make the opposition hated. And if you hate the enemy enough, you'll forgive your allies no matter how terrible they are or how little they serve your actual interests.
The perfect crime of modern partisan politics. Both sides do it, although in this case it's been taken to its nihilistic extreme by the Republicans. A month ago there were people literally saying on air that it's better to vote for a child molester than a Democrat. That's laying out the cards for everyone to see.