WASHINGTON • In an election campaign that has defied every prediction, there is at least one forecast nearly every one is comfortable making now that Mr Donald Trump and Mrs Hillary Clinton are the presumptive nominees: It will be one of the nastiest campaigns in recent US history.

Never mind the Trump hats screaming "Make America great again" or Clinton posters touting a leader "Fighting for us", everything that has happened so far indicates that this won't be an election decided by a defining vision of the future.

Rather, it will be one determined by how effective each candidate is at getting voters to dislike the opponent.

HIGH NEGATIVES

One huge reason for the high likelihood of impending ugliness is simply down to who the candidates are.

Mr Trump has repeatedly shown in recent months that he is a ruthless political dogfighter, willing to say nearly anything to get ahead.

And since he acknowledges that policy is not going to be his strong suit, he has consistently tried to keep the fight focused on rhetoric.

Nearly all of the stickiest moments of Mr Trump's young political career have involved nicknames and insults. Voters may not be able to explain the businessman's tax reform or healthcare plan but nearly everyone can recite his attacks against his rivals: former governor Jeb Bush is "low energy"; Senator Ted Cruz is "lyin' Ted"; Senator Marco Rubio is "lil' Marco"; Senator Elizabeth Warren is "Pocahontas" and Mrs Clinton is "crooked Hillary".

And if anyone believed that the bullying tactics would be toned down as he pivots towards the presidential election, Mr Trump made sure to dispel those notions this week with the way he responded to the mass shooting in Orlando.

While terror attacks typically trigger a period of national unity - consider how Democrats and Republicans put aside differences to give then President George W. Bush authority to pursue the culprits of the Sept 11 attacks in 2001 -Mr Trump has used the worst shooting in the country's history to divide.

He accused Democrats of not doing enough to battle the terror threat, congratulated himself for predicting that a terror attack would take place and went so far as to subtly imply that President Barack Obama had nefarious motivations when it came to the tragedy.

"He (Mr Obama) doesn't get it or he gets it better than anybody understands - it's one or the other, and either one is unacceptable," Mr Trump said in an interview with Fox.

That this was the tone of his response to tragedy spoke volumes of the man's political instincts and confirms that the scorched-earth policy deployed so effectively in the primaries is likely to become more entrenched.

After all, Mr Trump has not exhausted potential lines of attack against Mrs Clinton. He has only occasionally talked about her State Department e-mail scandal and hardly ever brings up the deadly attack on the US Embassy in Benghazi, Libya, in 2012 - two issues that his Republican colleagues consider the most damaging to Mrs Clinton.

Instead, he has road-tested some more outlandish claims. For instance, last month he spent a week accusing former president Bill Clinton of being a sexual predator and called Mrs Clinton his "enabler".

On her part, Mrs Clinton hasn't exactly been keeping above the fray either. Even if there was talk of running a positive, issues-based campaign, those plans appear to not have survived contact with the enemy.

In fact, observers said one of the best moments of Mrs Clinton's campaign so far was a speech she gave on June 3 which was focused on eviscerating Mr Trump's foreign policy. Rather than outline her own plans, Mrs Clinton repeatedly mocked Mr Trump, calling him "dangerous" and "temperamentally unfit" to hold office. "We all know the tools Donald Trump brings to the table - bragging, mocking, composing nasty tweets. I am willing to bet he is writing a few right now," she said that night.

That the two candidates appear so comfortable taking the low road is also largely down to how unpopular they are.

Polls put Mrs Clinton and Mr Trump as the two most unpopular candidates ever to run for president. Some 55.2 per cent of voters say they dislike Mrs Clinton, a number historically higher than that of any other presidential nominee other than Mr Trump. Some 61.5 per cent of voters had a negative opinion of the real estate mogul.

This level of distaste for either candidate lowers the risks one might normally associate with a negative campaign. Strategically, campaigns tread carefully when it comes to going overly negative because they fear independent voters might be turned off by attacks they consider below the belt or overly mean.

If, however, a lot of voters already clearly have a distaste for the candidate, then the chances of blowback from the negative campaign are minimised.

SORE LOSER

While the likelihood of a nasty campaign is practically a given, how all that nastiness will impact the outcome is a mystery.

There is a school of thought that believes that a dirty fight generally favours Mr Trump because he is very comfortable engaging in such battle. After all, every Republican rival that tried to go toe-to-toe with Mr Trump in the insult war ended up vanquished. As the old maxim goes: Never wrestle a pig in the mud; you both get dirty but the pig likes it.

At the same time, there are those who say that the likes of Mr Rubio and Mr Cruz were defeated because they were never prepared to commit to the same type of name-calling and insults that Mr Trump did. Everyone was uncomfortable straying too far from the traditional playbook.

Mr Rubio, for instance, did appear to get under the tycoon's skin for a few days with very personal attacks, but then became embarrassed by the negative campaign and apologised for his behaviour.

In contrast, Senator Elizabeth Warren has so far held up pretty well in her Twitter war with Mr Trump, returning every insult with insults of her own. She has at different times called him insecure, a fraud and a "thin-skinned racist bully".

It is difficult to predict how voters will react to all the nastiness. Conventional wisdom suggests that voter turnout drops when the negativity ramps up.

After all, negative campaigns have not been shown to be terribly effective at getting voters to switch sides. Rather, campaign strategists tend to use negative campaigns to discourage a particular group from voting. The basic idea behind negative campaigning in the US isn't about persuasion - insulting a candidate isn't going to make their supporters like you - it is about suppressing turnout by making voters who are leaning towards one's opponent dislike him or her enough that they stay home on polling day.

The big problem with such a strategy is what happens after someone wins.

A bitter, ugly election will exacerbate the already-significant disdain with which Republicans and Democrats regard each other, potentially making an already- gridlocked Congress more intractable.

If in all likelihood the next Congress is a divided one - with Democrats controlling the Senate and Republicans controlling the House - then a bloody election battle will ensure that the next administration will face legislative gridlock that is on a par with, if not worse than, what it is now.