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America is heading toward a historic presidential election, and that鈥檚 not only because it will likely feature the first woman candidate and a billionaire businessman who has never held elective office.

It also looks to be a matchup between two candidates with historically high unfavorable ratings 鈥� 鈥渁 battle of the negatives,鈥� if you will. Not since George H. W. Bush lost to Bill Clinton in 1992, has a presidential candidate had an unfavorable rating above 50 percent at this period in the election cycle.

Now it has two, with Donald Trump at 64 percent unfavorable and Hillary Clinton at 55,聽.听

What does that imply for the campaign tone for the next six months, and how will voters respond?

Because of the uniqueness of the campaign, experts can only guess. Will the high unfavorables motivate voters to stop the other guy? Or will sulking voters stay home?聽What does seem certain is that attacks will feature prominently in the campaign.

But not all attacks are equal, analysts add, and this election could sharply highlight the very different ways to run a "negative" campaign.听

Factual ads educate and motivate voters, while unsubstantiated attacks 鈥� especially personal smears 鈥撀爐urn them off and are 鈥渞isky,鈥� says Kathleen Hall Jamieson, director of the Annenberg Public Policy Center of the University of Pennsylvania.听

They particularly fall flat with female voters, who don鈥檛 like incivility in a campaign. Mr. Trump鈥檚 unfavorable ratings among women, which hit聽聽in April, raise questions about whether he can continue with his scorching rhetoric during the general election.

Recent days have shown the distinction in how Trump and Mrs. Clinton have gone negative.听

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Trump consistently refers to 鈥渃rooked Hillary,鈥� just as he used the labels 鈥淟yin鈥� Ted鈥� and 鈥淟ittle Marco鈥� to describe his primary opponents. In Oregon on Friday, he brought up Bill Clinton鈥檚 sex scandals, saying Mrs. Clinton was a 鈥渢otal enabler.鈥�

Clinton calls Trump a "loose cannon" and last week her campaign put out an聽聽video of GOP primary contenders and other prominent Republicans criticizing Trump. It concluded with Jeb Bush saying Trump 鈥渘eeds therapy.鈥澛�

The video reminds Professor Jamieson of the powerful 1964 ad that Democratic President Lyndon Johnson used against GOP conservative challenger Barry Goldwater in the general election. It, too, quoted sharp criticisms by Goldwater鈥檚 primary challengers.听聽

鈥淏oth Trump and Hillary are going highly negative, but there鈥檚 a sharp contrast between the two,鈥� says Norman Ornstein, a resident scholar at the American Enterprise Institute think tank. Almost everything from the Democratic side, he says, has been about issues. 鈥淵ou don鈥檛 have stuff from the National Inquirer and dirty things said.鈥�

The GOP primary belatedly taught Trump opponents such as Marco Rubio a lesson: to respond but not at the same level as Trump. Clinton looks like she鈥檚 learned that lesson, Jamieson says.听鈥淪he鈥檚 working hard to not sound unpresidential. She isn鈥檛 responding in kind.鈥�

So far, Trump has found that his broadsides bring rewards: large crowds, free media attention, primary wins, and cheers from many voters for his shoot-from-the-mouth style.

But they鈥檝e also turned off large swaths of voters, such as women and minorities, as well as people in his own party.听Trump could change his tone, Jamieson says, and then the question would be how quickly, or if, voters would forgive or forget.

The main driver for both candidates to go negative is weakness 鈥� their own, and their opponent鈥檚. Clinton, who is leading Trump in most polls, needs to make sure that his negatives stay high as a motivator for her base. And Trump needs to drive up Clinton鈥檚 negatives to unite Republicans against her, says Matt Mackowiak, a GOP consultant in Texas.

Could they drive down their own negatives with positive, advocacy ads about themselves? Voters are driven to the polls by traits such as 鈥渋nspiring,鈥� 鈥渃ares about individuals,鈥� and 鈥渧isionary,鈥� 聽show,聽but both front-runners score low on these characteristics.

Clinton and Trump will surely try to project positive images of themselves, experts say, 鈥渂ut for both of them, [the negatives] are so deeply baked in, it鈥檚 hard to see how鈥� they can substantially reverse their unfavorable ratings, says Republican pollster Ed Goeas.

鈥淭hey have to use the Jesse Helms strategy of 鈥榊ou may not like me, but I鈥檓 going to make sure you dislike my opponent more!鈥� 鈥� writes Mr. Goeas in an e-mail, referring to the late senator from North Carolina.

Gallup pollster Frank Newport cautions that unfavorable numbers can change over time. He points to Bill Clinton鈥檚 dramatic turnaround at the 1992 Democratic convention, for instance, while, by the fall, the senior Bush鈥檚 unfavorable rating was almost as bad as Trump鈥檚 is now.听

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What pollsters and others are unsure about, is whether two disliked candidates will drive high turnout. In 2008, Mr. Newport points out, Barack Obama generated high enthusiasm for his candidacy among certain groups, such as young people and African Americans. 鈥淚t鈥檚 possible it could work in the opposite direction,鈥� with certain groups such as Hispanics and women highly motivated to vote against Trump.听

On the other hand, voters deeply disappointed with the choices may stay home.听

鈥淵ou鈥檙e going to have a lot of complaining from voters that they don鈥檛 like the choices, the negativity. That the race isn鈥檛 worthy of the office,鈥� says Mr. Mackowiak.

He points to the clamoring for a third-party candidate from people such as Sen. Ben Sasse (R) of Nebraska, but says he has 鈥済rave doubts鈥� that such a candidate could succeed at this late stage.

The truth is, pollsters don鈥檛 know how voters will react to the battle of the negatives.听

鈥淢aybe both of them beat each other up and that raises the intensity on both sides. But it could have the effect of every day, voters just saying, 鈥業 don鈥檛 want this choice of the lesser of two evils,鈥� and just staying home,鈥� says Mr. Goeas.听

鈥淚t could go either way. We鈥檙e truly in uncharted waters.鈥�