3. Washington
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When asked if they trust the government, 89% of Republicans and 72% of Democrats say "only sometimes" or "never",
according to Pew Research. Six out of 10 Americans think the government has too much power, a
survey by Gallupsuggests, while the government has been named as the top problem in the US for two years in a row - above issues such as the economy, jobs and immigration, according to the organisation.
The gridlock on Capitol Hill and the perceived impotence of elected officials has led to resentment among 20 to 30% of voters, says polling expert Karlyn Bowman, from the American Enterprise Institute. "People see politicians fighting and things not getting done - plus the responsibilities of Congress have grown significantly since the 1970s and there is simply more to criticise. People feel more distant from their government and sour on it," she says.
William Galston thinks part of the appeal of Donald Trump and Bernie Sanders is down to frustration with what some see as a failing system. "So on the right you have someone who is running as a 'strong man', a Berlusconi and Putin, who will get things done, and on the left you have someone who is rejecting incrementalism and calling for a political revolution," he says.
Ted Cruz, who won the Republican caucuses in Iowa, is also running as an anti-establishment candidate. "Tonight is a victory for every American who's watched in dismay as career politicians in Washington in both parties refuse to listen and too often fail to keep their commitments to the people," he said on Monday night.
4. America's place in the world
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America is used to being seen as a superpower but the number of Americans that think the US "stands above all other countries in the world" went from 38% in 2012 to 28% in 2014,
Pew Research suggests. Seventy percent of Americans also think the US is losing respect internationally, according to a 2013 poll by the centre.
"For a country that is used to being on top of the world, the last 15 years haven't been great in terms of foreign policy. There's a feeling of having been at war since 9/11 that's never really gone away, a sense America doesn't know what it wants and that things aren't going our way," says Roberto Suro. The rise of China, the failure to defeat the Taliban and the slow progress in the fight against the so-called Islamic State group has contributed to the anxiety.
Americans are also more afraid of the prospect of terrorist attacks than at any time since 9/11, according to a New York Times/CBS poll. The American reaction to the San Bernardino shooting was different to the French reaction to the Paris attacks, says Galston. "Whereas the French rallied around the government, Americans rallied against it. There is an impression that the US government is failing in its most basic obligation to keep country and people safe."
5. Divided nation
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Democrats and Republicans have become more ideologically polarised than ever. The typical (median), Republican is now more conservative in his or her core social, economic and political views than 94% of Democrats, compared with 70% in 1994, according to
Pew Research. The median Democrat, meanwhile, is more liberal than 92% of Republicans, up from 64%.
The study also found that the share of Americans with a highly negative view of the opposing party has doubled, and that the animosity is so deep, many would be unhappy if a close relative married someone of a different political persuasion.
This polarisation makes reaching common ground on big issues such as immigration, healthcare and gun control more complicated. The deadlock is, in turn, angering another part of the electorate. "Despite this rise in polarisation in America, a large mass in the middle are pragmatic. They aren't totally disengaged, they don't want to see Washington gridlocked, but they roll their eyes at the nature of this discourse," says Paul Taylor. This group includes a lot of young people and tends to eschew party labels. "If they voted," he says, "they could play an important part of the election."