Kara Alaimo, an assistant professor of public relations at Hofstra University, is the author of " Pitch, Tweet, or Engage on the Street: How to Practice Global Public Relations and Strategic Communication ." She was spokeswoman for international affairs in the Treasury Department during the Obama administration. Follow her on Twitter @karaalaimo . The opinions expressed in this commentary are solely those of the author. View more opinion at CNN.

(CNN) It's going to get ugly out there. And there's a good reason for this.

Presidential candidate Joe Biden looked unprepared when Kamala Harris attacked his record on race during Thursday's Democratic debate. Harris, who is black, called Biden's comments about working with segregationist lawmakers "hurtful" and criticized his opposition to school busing, which she benefited from as "a little girl in California." Going on the offense paid off for Harris, who was widely seen as the winner of the night, while Biden was left to defend his record

Biden and his team should have been better prepared for the attacks, and voters should strap themselves in and prepare for worse as the 2020 campaign ramps up. The reason for this is simple: As the exchanges between Harris and Biden show, negative messages from politicians are much more effective than positive ones. Studies have shown such attacks receive more attention, are remembered longer and have a greater impact on voters' judgments than complimentary information they receive about candidates.

Vincent Covello writes in the " Handbook of Risk and Crisis Communication " that "communications that contain negatives -- e.g., words such as no, not, never, nothing, none, and other words with negative connotations -- tend to receive closer attention, are remembered longer, and have greater impact than messages with positive words." In fact, negative attacks are so damaging to a candidate that, according to Covello, it takes at least three positive messages to offset a single negative one.

A 2001 study of 24 years' worth of surveys on US presidential elections also found that negative messages influence voters' decisions more than positive ones.

The researchers noted that the reason for this could be that, for humans, taking stock of negative events -- which tend to be more irreversible than positive ones -- holds more survival value.

Political scientist Kathleen McGraw offered two other possible explanations for this so-called "negativity bias" in the " Oxford Handbook of Political Psychology ": "Negative information tends to be more diagnostic as to underlying traits and abilities, and it draws more attention because negative acts and attributes are less common than positive ones."

Voters pay attention to and are disproportionately influenced by attacks on politicians. All of this explains why even those who promise to run clean campaigns often end up going on the offense. While Bernie Sanders pledged not to run personal attack ads, he still ended up essentially attacking Hillary Clinton without naming her in his "Two Visions" ad during the 2016 race.

When Elizabeth Warren was asked by CNN about Biden's record on race, she declined to criticize him, choosing instead to say , "I'm here to talk about how to build a future." We'll see how long that lasts.

The fact is, candidates have every incentive to attack one another. This is unfortunate for Democrats, of course, as it could lead to infighting, expose cracks within the party, and threaten the standing of the eventual nominee, who may have a hard time uniting the party to defeat President Donald Trump after taking so many hits.

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That's why incumbent presidents often have an advantage: Sure, they still face challenges from fellow party members, but they can often run for reelection without suffering through bruising primaries. While former Massachusetts Gov. Bill Weld is mounting a primary challenge to Trump, the President has an easy road ahead compared to the 24 Democrats who will have to battle it out with one another in the months to come. And going on the attack clearly proved to be a winning strategy for Trump in the 2016 election, when he assigned vicious monikers to his competitors, like "Crooked Hillary" and "Little Marco."

Kamala Harris is experiencing a surge in the days after the first Democratic debates in large part because she went on the attack against Biden. It's unfortunate but true: For presidential contenders, disparaging other candidates can be a very effective strategy -- even though it can damage the party overall. That's why Americans should be prepared for the 2020 race to only get nastier.