It hasn’t always been this way. The last time Congress faced an impeachment debate, 20 years ago, five Democrats voted to impeach President Bill Clinton, also a Democrat. Thirty Democrats voted to open impeachment proceedings against him. Nearly 30 Republicans voted not to impeach the president.

Today, with the lone exception of Amash, not a single Republican has entertained the notion of impeachment against Trump, even though hundreds of former federal prosecutors, including some appointed by Republicans, said his actions as outlined in the Mueller report amount to a crime.

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What has changed? Tribalism. Polarization was pretty intense in the Clinton era, too, but today the parties have become less willing to accommodate any deviation whatsoever, and the issues that define each side are more entrenched — making someone like Amash stand out even more.

Congress, particularly the House of Representatives, is a mirror of the country. Voters are also becoming more polarized. Democrats and Republicans don’t really hang out anymore. Studies show they don’t even live in the same neighborhoods. So why would their representatives behave any differently?

And while polarization is inherent in both parties, it’s more present among Republicans today, said political science experts who spoke to The Fix. Republicans have Fox News and conservative talk radio, while the closest Democratic equivalent, MSNBC, isn’t nearly as doctrinaire.

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“They have more team spirit,” said David Karol, an expert in polarization at the University of Maryland.

Having such a dominant figure at the top of it all, President Trump, exacerbates the “you’re with us or you’re against us” mentality. In the Republican primaries leading up to the 2018 midterm elections, Trump helped unseat sitting Republicans — extremely conservative Republicans — who spoke out against him.

Former Republican senator Jeff Flake lamented this new normal when he briefly held up Brett M. Kavanaugh’s controversial nomination to the Supreme Court last fall. He said he would not have done what he did if he were running for reelection. Politics isn’t structured to reward compromise anymore, he said. “There’s no value to reaching across the aisle,” he told CBS’s “60 Minutes.” “There’s no currency for that anymore. There’s no incentive.”

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Amash is facing similar wrath. He tweeted on Saturday afternoon his belief that his party’s president engaged in “impeachable conduct.” By Sunday, he had a primary challenger.

Twenty years ago, during Clinton’s impeachment, things were different. There were more swing districts, and there was an understanding among party leaders that vulnerable members had to do what they had to do to represent their constituents and stay elected. The people who broke from their party had constituencies that were atypical from the rest of their party — one Democrat who voted to impeach Clinton, Ralph Hall, represented rural Texas, for example.

Some aisle-crossing still goes on today, but there’s less of it as the party’s tents get smaller and lawmakers are forced to take sides. Politicians have helped set it up that way, by drawing congressional districts to purposely be polarized to keep their party members safely elected. There are no Democrats representing rural Texas anymore.

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If you were to pick up that impeachment drama and drop it in today’s political climate, the Democrats who voted for articles of impeachment against Clinton probably wouldn’t even be considered Democrats. Some of the five have actually left the party and become Republicans since, Karol points out.

And the moderate Republicans who formed the coalition of lawmakers voting against impeachment are basically an extinct species. Voteview tracks the ideological range of lawmakers going back 20 years. In the era of the Clinton impeachment, there were some conservative Democrats and liberal-leaning Republicans. In today’s House of Representatives, there is no crossover, just a blank white space between the blue and the red.

Amash is one of the most conservative members in Congress, which is why his defection from the party line is such an anomaly, and why it struck such a nerve with Republican leadership. Republicans are willing to make an example of one of their own if it prevents others from joining him.

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"There is a strong ethos now in both parties that you've got to support your president,” said Matthew Green, a political science professor at Catholic University, who has written a book on the House Freedom Caucus.

Presidential loyalty is particularly important when your party’s in the minority and you need all the support you can get. “Having a president of the same party is especially valuable,” he said.

One interesting case study is Sen. Lindsey O. Graham (R-S.C.). He was in the House during the Clinton impeachment, and he gave a big speech about how “you don’t even have to be convicted of a crime” to be impeached. (He voted to impeach Clinton.) Today, Graham has totally changed his position. He says Trump shouldn’t be impeached because he didn’t commit a crime.

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Impeachment is inherently partisan, of course, and Congress is never blind to the party that the president belongs to. But Graham’s 180 suggests his reluctance to impeach is more about protecting the president than about some principled stand. It’s about protecting himself, too — Graham is up for reelection next year in a state where Trump is very popular, and like it is for so many members of Congress, the danger doesn’t come from the general election so much as from primaries.

Experts don’t see this tribalism lessening, unless there’s an earthquake-like shake-up to jolt people out of what they believe. Over the years, abortion, guns, immigration and even health care have all taken on a partisan sheen that defines people’s identities.