In homes and pubs, on leaflets and lampposts, debate is raging in Ireland over whether to lift the country’s decades-old ban on abortion.

Pro-repeal banners declare: “Her choice: vote yes.” Anti-abortion placards warn against a “license to kill.” Online, the argument is just as charged — and more shadowy, as unregulated ads of uncertain origin battle to sway voters before Friday’s referendum, which could give Irish women the right to end their pregnancies for the first time.

The highly charged campaign took a twist this month when Facebook and Google moved to restrict or remove ads relating to the abortion vote. It is the latest response to global concern about social media’s role in influencing political campaigns, from the U.S. presidential race to Brexit.

“We shouldn’t be naive in thinking Ireland would be immune from all these worldwide trends,” said lawmaker James Lawless, technology spokesman for the opposition Fianna Fail party. “Because of the complete lack of any regulation on social media campaigning in Ireland, somebody at the moment can throw any amount of money, from anywhere in the world, with any message — and there’s nothing anybody can do about it.”

The role of online ads in elections is under scrutiny following revelations that Russian groups bought ads on platforms such as Google and Facebook to try to influence the 2016 U.S. presidential race. Many of the ads were designed to sow confusion, anger and discord among Americans through messages on hot-button topics.

Few subjects are more emotive than abortion, especially in largely Roman Catholic Ireland. Despite the country’s growing diversity and liberalism — voters legalized gay marriage in a 2015 referendum — the result is expected to be close. The campaign is being watched, and sometimes influenced, by anti-abortion groups in the U.S. and elsewhere.

Voters are being asked whether they want to keep or repeal the eighth amendment to Ireland’s constitution, added in 1983, which commits authorities to defend equally the right to life of a mother and an unborn child. Abortion is legal only in rare cases when the woman’s life is in danger, and several thousand Irish women travel each year to terminate pregnancies in neighboring Britain.

Prime Minister Leo Varadkar’s center-right government backs lifting the ban and allowing abortion on request up to 12 weeks of pregnancy. Ireland is no stranger to referendums — this is its fifth in five years — and the country’s electoral laws regulate traditional forms of campaigning. Radio and television ads are banned completely, and foreign political donations are outlawed. But the 20-year-old electoral rules don’t cover social-media advertising, and there is no limit on campaign spending.

“It’s a complete Wild, Wild West,” said Craig Dwyer of the Transparent Referendum Initiative, a volunteer group set up to collect information on the ads being used to target Irish Facebook users. “When we started collecting this information there was absolutely zero regulation.”

The group has compiled and analyzed almost 900 Facebook ads connected to the referendum. Many were placed by registered lobby groups, and most came from inside Ireland. But several dozen were either untraceable or from overseas, including some that have been linked to U.S.-based anti-abortion organizations.