LONDON » There is no political advertising on television or radio in Britain. Fundraising and spending are strictly limited. Tight elections can turn on a relative handful of votes in a small number of competitive parliamentary constituencies.

So as Britain’s political parties head into a tight, unpredictable election Thursday, they are even more reliant than their U.S. counterparts on social media as a way to mobilize supporters for a last push and disseminate their messages directly to voters.

Social media makes up for "that small difference of being that tiny bit marginally better than the other party," said Anthony Wells, director of political and social opinion polling at YouGov, a prominent polling company here. Digital technology also helps parties winnow undecided voters from the rest of the electorate, he said.

The governing Conservatives, the opposition Labour Party and at least four smaller parties that could hold the balance of power are targeting disaffected young people who might not otherwise vote as well as undecided voters in critical districts.

They are drawing to some degree on the lessons of recent campaigns in the United States and in particular the digital expertise of President Barack Obama’s 2008 and 2012 campaigns. The Conservatives are being advised by Jim Messina, who as Obama’s 2012 campaign manager set a new standard for the use of data and digital technology, and Labour by David Axelrod, Obama’s communications guru.

Prime Minister David Cameron got off to a rocky start on the issue of social media. He told a radio interviewer in 2009, a year before he became the nation’s leader, that he was not on Twitter because its "instantness" did not sit well with a politician’s need to think about what he says – and then used a profanity to punctuate a point about "too many twits." But he, or his campaign, has since shifted course, with more than 1,700 posts made from David—Cameron, which has more than 900,000 followers.

But social media, almost by definition, mocks any efforts by candidates to control their message or their image. On Friday, when Cameron inadvertently referred to the election as "career defining" – he quickly corrected himself to say "country-defining" – the response on Twitter was predictably harsh.

On the other side, the Labour Party leader, Ed Miliband, suddenly found himself something of a Twitter sensation among admiring young women who made milifandom a brief but publicity-heavy trend in the campaign’s closing weeks.

The smaller parties have also embraced innovative ways to promote themselves. The Green Party had what might have been the breakout hit of the campaign with a video spoofing the leaders of the bigger parties as members of a boy band all singing the same tune.

Judging by the bills, Facebook is a vital campaign tool for the Conservatives. British news organizations have published invoices showing that the Conservatives have been spending about 110,000 pounds ($170,000) a month on Facebook ads (Obama spent $78 million on online ads for his 2012 re-election bid). Party spokesmen declined to comment on the strategy.

Labour and the Liberal Democrats, the two other main parties, use sites like Facebook to a lesser extent, they say, because they have less money than the Conservatives. Instead, they have invested in software like NationBuilder, which allows campaigns to filter undecided voters and alert potential supporters by matching electoral rolls and their online activity.

Created by a startup in Los Angeles, NationBuilder is credited with helping the Scottish National Party win the 2011 parliamentary elections there and in aiding the Liberal Democrats in an important midterm parliamentary race in 2013.

The parties are also using Twitter, which unveiled a postcode-targeting technology in time for the election, allowing advertisers to direct their posts at users in specific locations.

Smaller parties like the Green Party and the anti-immigration U.K. Independence Party are relying on the "free" viral power of social media to try to burnish their appeal as grass-roots organizations and to make up for modest campaign budgets.

The Green Party said it had used crowdfunding to field candidates across 75 seats. An online petition signed by 280,000 people propelled its leader, Natalie Bennett, onto a television debate featuring party leaders after the broadcaster initially ignored her. The apparent snub helped bump up party membership to more than 60,000, a level unseen since 1989.

Douglas Carswell, a UKIP member of Parliament who was a Conservative, is seen by many as the most social-media-savvy politician in Britain. Social media "allows you to communicate with voters in a nonparty-political way," he said one recent morning in Clacton, where he is running for re-election.

But he barely mentions UKIP in his Twitter feed, he said, a habit he began in 2009 after ignoring directions from the Conservative Party about when and what to post. More than a third of his posts are replies to users, according to Twitonomy, a Twitter analytics and monitoring tool. By contrast, just 11 percent of posts from Cabinet or shadow cabinet politicians were replies to Twitter users.

Carswell also blogs, uses Facebook and responds personally to emails from constituents. "In an age where people feel that big corporate parties have been captured by big vested interests, I think that’s actually quite a refreshing thing," he said.

But he acknowledged the limits to social media’s ability to sway undecided voters. "What might do, though, is if I can knock on people’s doors and talk to them, and social media allows me to do that," he said. "That’s the great paradox. Digital technology gives you the tools to have not a campaign in cyberspace, but actually a genuine grass-roots campaign."

Campaign strategists and analysts share his doubts about how much social media influences voting because, they say, Britons generally do not like discussing politics with friends or colleagues, whether in person or on Facebook.

"Tweets don’t win elections, people win elections," said Matthew McGregor, the Labour Party’s chief digital strategist and political director of Blue State Digital, a consulting firm. He ran a unit of Obama’s digital campaign in 2012 that crafted viral attack videos and Twitter posts. "Our online campaign is there to help the field operation be stronger and more and more efficient, and that’s our focus," he said.

Twitter and email help rally volunteers and activists, who are at the heart of Labour’s campaign strategy, he said.

Jack Thorn, 46, was among about 30 Labour supporters who gathered in a district in northern London one recent morning to knock on the doors of undecided voters. "All we’ve got are people on the ground, being polite, trying to represent the party," he said. "That’s our strength."

Sharvani Mukherjee, 27, opened the door to his knock. She planned to vote, she said, but was still undecided. Despite being an avid user of Instagram and Facebook, her political views are shaped from watching television debates and reading the news, she said, "not what people talk about on Twitter."

Kimiko de Freytas-Tamura, New York Times

© 2015 The New York Times Company