The potentially deadly bombs delivered to Hillary Clinton, Barack Obama and other prominent Americans failed to detonate.

Instead they set off a raging tribal battle, a wild rush to blame that included conspiracy theories, angry editorials and vituperative tweets.

On Twitter, U.S. President Donald Trump attacked the media.

"A very big part of the Anger we see today in our society is caused by the purposely false and inaccurate reporting of the Mainstream Media that I refer to as Fake News," he tweeted.

He's right about one thing — anger is pervasive.

In fact, when Americans vote in the midterm elections, the most significant factor driving them to the polls won't be party loyalty or the appeal of their candidate. Nor will it be an issue like healthcare, taxes, the deficit, guns, the Mueller probe, immigration, foreign policy or defence.

Scientists say the number one motivating factor in American democracy now is anger, and it has been for some time.

"Since the 1980s … partisans have expressed more dislike and more negativity towards the opposing party," Masha Krupenkin said on Day 6.

"But it's really accelerated in the past two decades."

Hate drives the vote
Krupenkin is a political scientist at Stanford University. In a study she co-authored called  "The Strengthening of Partisan Affect," Krupenkin says that "as animosity toward the opposing party has intensified, it has taken on a new role as the prime motivator in partisans' political lives."

That means hatred of an opponent is more likely to drive a voter to the polls than enthusiasm for their own candidate or a commitment to a goal or ideal.

Krupenkin writes that, "today it is outgroup animus (hatred of the opponent) rather than ingroup favoritism (loyalty to your candidate) that drives political behavior."

In other words, in the U.S. today, hate drives the vote.

These are not opinions, but a scientific assessment.

Krupenkin's study tracked 30 years of survey data, performed analytical interviews and used a research tool called a feeling thermometer to gauge the warmth — or hatred — a subject feels for a candidate.

"It's actually a very well-known tool," Krupenkin explained. "We asked people on a survey, 'from zero to 100, how do you feel about a candidate or a person or a group?'"

Complete indifference would be 50, but that's not the number Krupenkin's subjects landed on.

"The most popular response for people of the opposing party was zero," she said. One in five respondents expressed undiluted hatred for their opponent.

That didn't used to be the case.

"Until fairly recently, actually, the most popular answer was 40," Krupenkin said. "So that was, you know, sort of mild dislike. But it's really shifted over the course of the past decade or two."

It could also change just about everything about politics.

Rise of the despicable
If votes are cast not to support a platform or policy, but to block an undesirable from office, it brings fundamental instabilities to the political system.

"When we have politicians, you know, whose supporters are driven by how negative they are about the opposing party, we end up in a system where politicians aren't really trying to propose policies that help their constituents, but rather sort of competing for how to bash the opposing party the most," Krupenkin said.

Winning becomes more important than governing, and bipartisanship dissolves in gridlock.

"In fact, the incentive is to just continue to push on this anger and this negativity, which in turn creates the sort of self-reinforcing cycle," Krupenkin said.

It also leads to an accountability gap.

"When negative affect for the opposing candidate becomes the primary motive underlying vote choice, candidates seeking their party's nomination are less likely to be sanctioned for demonstrating incompetence, dishonesty, and unethical behaviour," she writes.

That may explain why Roy Moore nearly won a seat in Alabama in spite of allegations of child abuse.  

Krupenkin says the system is now gamed to reward a toxic personality over a candidate displaying trust or likeability. Despicable people have an advantage.

"It would certainly demand that a certain kind of politician — someone who does feel empowered by being hateful — would put themselves forward," she said.

The tribal divide
Earlier this month, after the confirmation of Supreme Court nominee Brett Kavanaugh, both sides of the aisle sought to mobilize the anger stoked during the process.  But that process didn't so much divide the country as reveal fault lines that have long been present.

Krupenkin says the barriers that divide Americans are getting stronger.

"We know, for example, that intermarriage rates between people of the opposing parties are down," she said.

"We know that there have been some studies on online dating where they show that people are really not likely to date people of the opposing party. And we also know that in social media, people tend to self-segregate in groups, the sort of echo chambers."

Expect those groups to vote along party lines in November. As partisan identity fuses with social identity, rational and critical thought fades from politics.

"Partisanship — and just a lot of other very important salient identities like race and gender — are these very core identities that are a part of who people are," Krupenkin said.

"And it's very difficult to get people to think rationally about these sorts of things."