It's becoming increasingly clear that the fall presidential campaign – featuring the likely matchup between Democrat Hillary Clinton and Republican Donald Trump – will be one of the most negative ever. Historian Douglas Brinkley predicts that the general election will be "incredibly nasty, unbelievably nasty."

The two front-runners are so far ahead that it's unlikely they can be derailed as the nominees. But they suffer from massive unfavorable ratings. Fifty-seven percent of voters have an unfavorable view of Trump and 52 percent have an unfavorable view of Clinton, according to the latest CBS/New York Times poll. Only 24 percent have a favorable view of Trump compared to 31 percent for Clinton.

These are the highest unfavorable ratings of any major candidates measured by CBS since 1984, when the network began asking the question.

Half of all voters said they would be scared if Trump were elected president and an additional 19 percent said they would be concerned. Thirty-five percent of all voters said they would be scared if Clinton were elected president and 21 percent said they would be concerned. These are not reassuring numbers for either candidate.

Such a negative environment encourages candidates to tear each other down because it's so difficult to make their images more positive. The goal is to make one's opponent look worse. And that's what's happening.

Clinton and Trump gave separate speeches to the American Israel Public Affairs Committee, a pro-Israel group, in Washington this week, suggesting how tough the fall campaign will be. Clinton blasted Trump for wavering in his support for Israel. "We need steady hands," she said, "not a president who says he 's neutral on Monday, pro-Israel on Tuesday and who-knows-what on Wednesday." Trump told the same crowd he backs Israel fully, a change from recent comments that he would be "neutral" in negotiating a peace agreement between the Israelis and Palestinians. He added: "Hillary Clinton, who is a total disaster by the way, she and President Obama have treated Israel very, very badly."

Trump says he hasn't started really attacking Clinton yet. He promises to begin that process in earnest soon after he locks up the GOP nomination and doesn't have to worry about his remaining Republican rivals, Sen. Ted Cruz of Texas and Gov. John Kasich of Ohio.

The United States has seen very negative campaigns before, suggesting that dirty politics is a powerful force in the nation's bloodstream. It's been that way almost from the beginning of the Republic. One of the worst campaigns was in 1800. That was a contest between two of the iconic leaders of the day, both of them founders of the new nation, President John Adams and challenger and Vice President Thomas Jefferson. An Adams surrogate said if Jefferson became president "we would see our wives and daughter the victims of legal prostitution." A Connecticut newspaper said Jefferson would create a nation where "murder, robbery, rape, adultery and incest will openly be taught and practiced." On the other side, partisan journalist James Callender wrote that Adams was a "repulsive pedant" and "gross hypocrite" who "behaved neither like a man nor like a woman but instead possessed a hideous hemaphroditical character."

This year's campaign among the Republicans has descended to similar lows, with Trump insulting his adversaries or belittling them in vulgar terms. His targets have included women, Mexicans, Muslims and protesters. He has made below-the-belt references praising the size of his private parts. He has contributed to an atmosphere of tension and in some cases violence at his rallies by berating anti-Trump protesters and urging his supporters to confront them.

Today's media are drawn to conflict and harsh attacks and this in turn encourages one cycle of negativity after another.

Looking ahead, Clinton allies see many vulnerabilities in the Donald which they plan to attack, such as his corporate bankruptcies, how he treated everyday people in running his businesses, and the idea that in his career he has emphasized making money and promoting himself above all rather than strengthening communities and helping families improve their lives. The Clinton forces say he has no real convictions that can be counted on.

For his part, Trump has threatened to venture into very volatile territory, arguing that the sexual transgressions of Bill Clinton while he was president, and for which he was impeached, are fair game. Trump also says Hillary Clinton lacks the stamina and strength to be an effective president.

There is also the likelihood that the fall campaign will divide the country on the basis of race, gender and economic class.

Aiming at Trump, President Obama recently said, "In America, there aren't laws that say that we have to be nice to each other, or courteous, or treat each other with respect. But there are norms. There are customs. The longer that we allow the political rhetoric of late to continue, and the longer that we tacitly accept it, we create a permission structure that allows the animosity in one corner of our politics to infect our broader society. And animosity breeds animosity."