The "Brexit" debate that finally culminates in a vote on Thursday has been hard-fought, turning advertising, social media and traditional media into a battleground for those who want Britain to remain part of the European Union and those who want out.

Although voters as a whole seem split, the vast majority of the advertising and marketing industry supports staying, with WPP CEO Martin Sorrell perhaps the industry's most vocal "Remain" supporter. They are backing the campaign officially known as Britain Stronger in Europe, facing off against the organization Vote Leave.

The arguments have focused on the economy and immigration, with politicians on both sides making predictions that are often difficult or impossible to prove. Advertising has necessarily simplified the cases, using hard-hitting images that invite a gut response from voters.

The official campaigning bodies for each side are only legally allowed to spend £7 million, the equivalent of approximately $10 million, but with social media playing such a key role, the restraints have hardly kept the volume down.

Adam & Eve/DDB's effort showed a mocked-up image of the three most prominent "Leave" campaigners in a casino, next to the headline, "Don't Let Them Gamble With Your Future."

James Murphy, co-founder and CEO of Adam & Eve/DDB, said, "'Vote Leave looked at Trump and saw that if you campaign on a popular angle you can tell huge lies. It's the dawn of post-truth politics. This ad shows a simpler emotional truth and stays away from statistics trading."

M&C Saatchi is also working with Britain Stronger in Europe. Its final ad showed an open door leading into a black corridor with the words, "Leave. And There's No Going Back."

"It's aimed at younger voters, asking them to think carefully because this is forever," M&C Saatchi Worldwide CEO Moray MacLennan said. "Of the over 55s, 80% will vote and most are pro-Brexit. The 18-to-34 age group are the key Remain supporters, but only 50% are planning to vote. We've had to focus on mobilizing the vote."

M&C Saatchi also created a poster showing chief Brexiters Boris Johnson, the former mayor of London, and Nigel Farage, leader of the U.K. Independence Party, sitting together on the branch of a tree, sawing furiously to detach themselves from the trunk. Mr. MacLennan said, "We needed to show that those two are in it together, to demonstrate the company they are keeping."

Without the widespread backing of the communications industry, the "Vote Leave" campaign has had fewer political ads in its armory. Its most powerful image so far -- produced by Mr. Farange's UKIP party --was a giant poster of a queue of migrants next to the words "Breaking Point." Opponents likened the poster to Nazi propaganda and reported it to the police for inciting racial hatred.

But a group of pro-Brexit communications professionals including Tim Bell, a co-founder of Saatchi & Saatchi best known for his work getting Margaret Thatcher elected as prime minister, has assembled to work together under the Communicators for Britain banner.

On the final day of campaigning, the agency London Advertising released a series of ads, one of which attempts to counter racist accusations by showing a picture of an Indian supporter who says, "This is not a racist thing. This is not an immigration thing. This is about having faith in yourself and hope for the future."

Many agencies have taken matters into their own hands, adding their individual voices to the campaigning.

Adam & Eve/DDB became so involved that it had to register as a campaigning organization. U.K. rules allow a spend of up to £10,000 for non-official bodies, nearly $15,000, but the figure includes the value of time spent, so the agency registered to avoid risking breaking the law.

The agency enlisted some A-list names, including actress Keira Knightley and physicist Stephen Hawking, to star in a series of hard-hitting video ads that urge people to vote in the referendum by warning, "Don't let others fuck with your future."

Although they don't outwardly take sides, the videos are clearly aimed at young people and, by mobilizing them to vote, keeping Britain in the EU.

Targeting Cannes attendees
U.K. delegates at the Cannes Lions International Festival of Creativity have no excuse to miss the referendum. Adam & Eve/DDB also created an ad directly aimed at industry types, to make sure they registered for a postal vote before leaving for the French Riviera. Using a bottle of rosé as its motif, the ad read, "Off to Cannes? Don't forget: Factor 30; your dignity; your postal vote for the EU referendum." A registration form was attached.

"We didn't want it to look like an ad that was trying to win an award," Mr. Murphy said. "It's practical. The main thing is the form."

Voters are not allowed to take pictures in the actual polling booths, but postal voters have been able to photograph their polling cards and show where they put their mark. Such photos have been all over social media, as voters make their allegiances clear in a way that hasn't happened in U.K. general elections.

Crispin Porter & Bogusky London transformed the window of its busy Kings Cross offices into a "Game of Thrones"-themed call to action. Passers-by could see two windows, one with a "Stronger Together" banner and the words, "If the North left the Seven Kingdoms you'd have an opinion." The other banner read "Better Off Out," above the words, "Whatever kingdom wins, have your voice heard."

Agency Rufus Leonard has made a film championing a positive and emotional case for staying in the EU, in an attempt to appeal to younger voters who are turned off by the tide of negative, rational, political campaigning.

Saatchi & Saatchi London has been working with Operation Black Vote, or OBV, an organization devoted to getting black, Asian and other ethnic minorities to register and vote. Only half of those groups' members are statistically likely to vote.

It created a poster subverting a famous picture of the Bullingdon Club, an exclusive, all-male dining club at Oxford University where Prime Minister David Cameron and Mr. Johnson were famously members. Instead of the familiar young, all-white line-up, the "members" are diverse in age, skin tone and gender.

"OBV contacted us without a doubt because our CEO is black and we are involved in diversity issues," said Richard Huntington, chief strategy officer of Saatchi & Saatchi London Group. "We had to simplify a complex issue and start a bit of a fire. The image is traditionally one of privilege, of a group who were born to rule. But voting is everyone's birthright. You take power when you register. In a referendum, every vote is equal and massive."

The drive to get people to register was so successful that the government's website crashed, and the deadline had to be extended. "I like to think we played our part in that," Mr. Huntington said.

Saatchi & Saatchi has also collaborated with campaigning comedian Eddie Izzard and pressure group We Are Europe to push the message that there would be no going back if the U.K. votes to leave. They have created a tattoo shop in London's Soho neighborhood offering free satirical Brexit tattoos with the warning that they come "at the price of permanency."

In another effort to get young people out to vote, Channel 4's entertainment brand, E4, created the "Referendum in Chelsea" video, in which four ridiculously posh members of popular reality series "Made in Chelsea" discuss key issues surrounding the debate. Binky, Jamie, Mark-Francis and Toff wonder if Brexit would affect their ability to get hold of a good Pinot Noir, and whether the U.K. would still be able to compete in the Eurovision Song Contest. The friends conclude that they need to make their vote count.

A Marketing Society survey found that 77% of communications industry workers favored staying in the EU, citing short-term business confidence (94%), ease of travel (88%) and political stability (79%) as the key reasons for remaining.

Individual marketers have spoken out, too: John Lewis Chairman Charlie Mayfield has said a Brexit vote would have an "adverse impact" on consumer confidence and the economy for up to five years.

And when the Institute of Practitioners in Advertising surveyed its members, 88% said they thought their business prospects would be better if the U.K. remains in the EU. ZenithOptimedia forecast that leaving the EU would result in a long-term slowdown of economic growth, costing the U.K. £70 million in ad spending growth each year and a total of £1 billion by 2030.