Matthew Yglesias has an important piece marred only by the risible neologism “buckraking.”
Language crankiness aside, this is a critical thing to understand about dysfunctions in politics, or, honestly, the human condition in general. The way it does is as follows:
Suppose that someone that you don’t trust tries to influence you to go against your principles. They might try reasoned debate, emotional appeal, quid-pro-quo, peer pressure, bribery, blackmail, or threats. And they might succeed! If you have never folded and done something you shouldn’t because someone influenced to, you’re a saint or a liar. But the thing about this is that if you usually recognize that you’re making a decision that you ordinarily wouldn’t, and probably you don’t change your principles. On a political level, you’re supposed to make this kind of compromise. It’s how things get done, and that’s fine.
Much more insidious is when someone you do trust, and trust implicitly, tries to influence you to go against your principles. You apply a lower level of judgment when the influencer is on your list of friends. You’re not just more likely to agree to their pernicious request, you’re also (troublingly!) much more likely to internalize their judgment and defend their request as not a compromise you made for a relatively good reason, but as the right thing on its own merits.
This is a general problem with group identification. I think that a lot of people identify as “Republicans” because they believe in personal responsibility, limited government, and economic freedom. And a lot of people identify as “Democrats” because they believe in helping disadvantaged groups like the poor, women, and minorities. And both of those belief systems are fundamentally reasonable and, indeed, laudable. But when your identity includes “Republican,” it becomes much easier for constituencies in the Republican party to get you to support military adventurism, torture, and unlimited support for plutocrats. And when your identity includes “Democrat,” it becomes much easier for constituencies in the Democratic party to get you to support, well, military adventurism, standing up for the rights of teachers over schoolchildren, and as Matt points out unlimited support for plutocrats when it comes to intellectual property rights.
Which is one reason why you should be very careful of which groups you include as part of your identity.