Protesters shout at Donald Trump as he speaks during a campaign event at the CFE Federal Credit Union Arena in Orlando, Fla. (Jabin Botsford/The Washington Post)

Each week, In Theory takes on a big idea in the news and explores it from a range of perspectives. This week, we’re talking about polarization in politics. Need a primer? Catch up here.

Alan I. Abramowitz is the Alben W. Barkley professor of political science at Emory University. He is author of “The Disappearing Center: Engaged Citizens, Polarization and American Democracy” and “The Polarized Public: Why American Government Is So Dysfunctional.”

It is presidential primary season, and just as in 2008, the Democratic and Republican candidates sound as though they are talking to two different countries. Only this time, the divide between those two countries has grown much larger — today it is fueled by increased racial and cultural friction.

Republican candidates are talking to a country that is increasingly angry and fearful — angry at a president they view as a dangerous radical who seeks to weaken America, and fearful of threats posed by jihadists from the Middle East and illegal immigrants from Mexico. Their target country is overwhelmingly white, mostly male and relatively old. Its population is concentrated disproportionately in small-town and rural America and is shrinking in size with every election cycle. The candidate who has clearly captured the mood of this country in 2016 is Donald Trump, which is why he is now the favorite to win the Republican presidential nomination.

Democratic candidates are talking to a country that is more hopeful than angry but that is increasingly worried about the effects of growing wealth concentration, declining economic opportunity, racial injustice and the threat of climate change, and that is frustrated at the inability of our government to deal with these challenges. That country is racially diverse, relatively young and heavily female. Its population is concentrated disproportionately in large metropolitan areas. It is growing in size with every election cycle, but its citizens don’t turn out to vote as consistently as the citizens of the other country, especially in midterm elections. Bernie Sanders’s message clearly resonated with many of the younger citizens of the Democratic nation, but Hillary Clinton’s appeal to African Americans and Latinos who make up almost half of its voters appears likely to carry the day in the end.

The citizens of these two nations disagree about many things, including the Affordable Care Act, environmental regulation, immigration reform, abortion and same-sex marriage. But what may be even more significant than policy disagreements is the fact that citizens of these two nations look at each other with deep suspicion and hostility. Those on the other side are no longer viewed as mere political rivals but as enemies.

Jeers and violence erupted between Donald Trump supporters and protesters at the Republican frontrunner's rally in Fayetteville, N.C., on March 9. (Jenny Starrs/The Washington Post)

These attitudes are increasingly found among supporters of both major parties, but they are clearly more prevalent today among Republican voters. This may be partially due to the fact that a president from the opposing party provides a high-profile target for partisan attacks. But there is no doubt that anger and fear have been cultivated by GOP leaders and candidates as a tool for energizing and mobilizing the party’s base in elections. This year, however, Donald Trump has turned that anger and fear against the Republican establishment itself — effectively arguing that the party’s leaders have failed to deliver on their promises to reverse President Obama’s policies and return the nation to a mythical golden age.

One cannot understand the rise of Donald Trump without taking into account the growing racial, cultural and ideological divide between the two major parties. Trump’s message, that he is the only one who can provide the strong leadership needed to overcome the threats posed by illegal immigrants at home and terrorists abroad, plays directly to the anger and fears of a large part of the Republican nation.

But a Trump nomination would also alienate a substantial minority of Republican voters who find both his message and his personal style repellent while uniting Sanders and Clinton supporters within the Democratic nation. Thus a Trump candidacy would almost certainly ensure the election of Clinton and greatly increase the odds of Democrats regaining control of the Senate and making substantial gains in the House.

It might take just take such a devastating defeat to cause Republican leaders to take seriously the message of their party’s post-2012 election autopsy. In a nation that is becoming more racially diverse with every election cycle, the GOP cannot remain a viable party in national elections by doubling down on angry white voters. Such a change in direction might begin to bring the two nations back together.

Explore these other perspectives:

David Broockman: How well-meaning political reformers are helping to elect President Trump

Thomas E. Mann and Norman J. Ornstein: Republicans created dysfunction. Now they’re paying for it.

Dana Nelson: The growth of executive power has turned politics into war

Jim Marshall: Congress could reduce polarization. It has chosen not to.

Thomas Petri: Our government is messy — but that doesn’t mean it isn’t working

Jane Mansbridge: Three reasons political polarization is here to stay