If there are any doubts that political polarization is at record highs, consider the tenor of the 2016 White House race so far.

Hillary Clinton once named Republicans as the enemy she's most proud of. Chris Christie recently called President Obama a "feckless weakling." Donald Trump has called Mr. Obama "the worst president ever" and Mrs. Clinton "the worst secretary of state in the history of the United States."

Candidates on both sides of the aisle have spared no insults to whip up antipathy, and with it, they hope, support.

But South Carolina Sen. Lindsey Graham, who is running for the Republican presidential nomination, is hitting back at the divisive climate.

“To those people that believe Obama is a Muslim and was born in Kenya, I’ve lost ya a long time ago,” Senator Graham said Wednesday on CNN's New Day, the morning after the fifth GOP presidential debate. “There’s a dislike for President Obama in my party that’s unhealthy. There was a dislike for President Bush in the Democratic Party that was unhealthy. He is my president."

His remarks underscore the fever pitch political polarization has reached in recent years, says Peter Hanson, assistant professor of political science at the University of Denver.

“Democrats and Republicans have not been this polarized in nearly 100 years," says Professor Hanson. "Today’s politics are rooted in deep societal changes since the 1960s and they won’t be going away anytime soon."

Indeed, Republicans and Democrats are more divided along ideological lines, and partisan antipathy is deeper and more extensive, than at any point in the last two decades, according to 2014 data from the Pew Research Center.

In 1994, 17 percent of Republicans had very unfavorable opinions of Democrats and 16 percent of Democrats had very unfavorable views of Republicans, according to Pew. Since then, the contempt has more than doubled: By 2014, 43 percent of Republicans and 38 percent of Democrats viewed the opposite party in strongly negative terms.

So intense has the antipathy grown that 27 percent of Democrats and 36 percent of Republicans see the opposing party as "a threat to the nation's well-being."

A number of factors are behind the polarization, which has grown under both Presidents Bush and Obama.

An increasingly partisan news media and blogosphere that amplifies conflict and contention is partly to blame, says Gallup’s Jeffrey Jones.

“These increasingly partisan views of presidents may have as much to do with the environment in which these presidents have governed as with their policies, given 24-hour news coverage of what they do and increasingly partisan news and opinion sources on television, in print and online,” Mr. Jones wrote in a Gallup report earlier this year.

Demographics also may play a role. As the share of nonwhite voters has grown since the 1960s, so too has the racial disparity between the two parties. Nonwhite voters made up 10 percent of Mitt Romney voters, according to the 2012 national exit poll. In contrast, they comprised 45 percent of all Obama voters in 2012.

"The growing dependence of Democratic candidates and office-holders on nonwhite voters, along with a Republican strategy of appealing to white voters unhappy with the Democratic Party’s racial and economic liberalism," has served to further entrench the divide, writes Emory University political scientist Alan Abramowitz for The Washington Post.

And an electorate increasingly marked by fear and anger hasn't helped, adds The Christian Science Monitor's Peter Grier.

"Fear and anger are likely causes of much of this sorting," he writes. "Many voters aren’t so much trying to elect their candidates as block the ones from the other party, whom they see as a danger to the republic. Negative partisanship has become one of the strongest forces in this particle physics theory of US politics."

These pitched levels of polarization are dangerous for the country, says Melody Crowder-Meyer, an assistant professor of politics at Sewanee, the University of the South, in Sewanee, Tenn.

"When members of the two parties don’t just disagree, but fundamentally believe that people from the other party are evil, unpatriotic, and the like, it makes it difficult to compromise," she says. "The American political system, with its checks and balances, requires compromise across party lines and across branches of government to get things done. So, in a time of polarization ... we will be more likely to see legislative gridlock, executives turning to unilateral action to get things done, and a less effective governing system overall."

But things may not be as dire as they appear, says La Trice Washington, an associate professor of history and political science at Otterbein University in Westerville, Ohio.

"I believe the appearance of extreme polarization is a greater perception than it is a reality," says Professor Washington. "What you are seeing now is that we are at the height of election and the loyalists are who the politicians are speaking to."

"With advent of social media and proliferation of it, it makes us look extremely divided," she adds. "Is there division, yes, but I think it's a bit overblown."

The Pew report backs that up.

Perceptions of partisanship are skewed because "many of those in the center remain on the edges of the political playing field, relatively distant and disengaged, while the most ideologically oriented and politically rancorous Americans make their voices heard through greater participation in every stage of the political process," the report suggests.

In fact, "these sentiments are not shared by all – or even most – Americans. The majority do not have uniformly conservative or liberal views. Most do not see either party as a threat to the nation. And more believe their representatives in government should meet halfway to resolve contentious disputes rather than hold out for more of what they want."

Across-the-aisle, unity-building gestures by politicians, such as the one Mr. Graham made Tuesday night and Wednesday morning in his CNN interview, offer a blueprint for a path to unity.

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Speaking at the undercard debate, Graham apologized to people of Muslim faith worldwide for Donald Trump's proposal to bar all Muslims from entering the United States.

“I am sorry – he does not represent us," Graham said at the Venetian Hotel in Las Vegas. "What he said about banning Muslims has made us all less safe."