LONDON—The Manchester suicide bombing has led to the suspension of the campaign for the U.K. general election, as Prime Minister Theresa May’s party saw its sizable lead in the polls eroding.

Political parties said they would pause their campaigning for the June 8 vote, with Labour leader Jeremy Corbyn announcing it “will be suspended until further notice.” The Scottish National Party was scheduled to launch its manifesto on Tuesday but postponed it following the attack.

Mrs. May called the election on April 18, hoping her ruling Conservative Party would benefit from the huge lead in the opinion polls they were enjoying over the main opposition Labour Party.

But in the past week, support for her party eroded following publication of its manifesto. On Monday, she backtracked on a proposal in the manifesto to levy a charge on people’s houses after they died to pay for their social care.

Opposition politicians had described the proposal as a “dementia tax.” The manifesto was also criticized for not laying out the costs of its proposals.

Heading into the campaign, Mrs. May had a double-digit lead over Labour’s Mr. Corbyn in the polls. That lead shrunk to a nine-point gap over the weekend as voters fretted over the proposal on health care for the elderly, YouGov said.

“She has had the most difficult week since the publication of the manifesto and has looked at her most vulnerable since arriving in Downing Street in July,” said Mujtaba Rahman, head of the Europe practice of the Eurasia Group consulting firm.

“All of that will now be overtaken by yesterday’s events and will refocus the campaign on the issue of leadership,” he said.

That new focus will allow Mrs. May to draw attention back to her campaign mantra of “strong and stable leadership” and emphasize the supposed contrast with Mr. Corbyn.

The campaign pause could give the prime minister an opportunity to regain her footing and burnish her leadership credentials, pollsters say.

“When one has crises of this kind, they tend to favor the government in office,” said Wyn Grant, professor of politics at the University of Warwick. “They can reassure people and show people they are in control.”

Mrs. May previously dealt with terrorism matters as head of the U.K.’s Home Office, giving her experience that can “equip her to deal with this,” he added.

Still, analysts cautioned much would depend on how Mrs. May handles the fallout of the attack, details of which still remain unknown. She said she would travel to Manchester and host an emergency meeting on the bombing on Tuesday.

The bombing in Manchester came two months after Khalid Masood, a 52-year-old British-born man, drove a car into a crowd of people near the British parliament and stabbed a policeman, leaving five dead.

Past examples of British elections being disrupted by violence are rare. In 1979, Shadow Northern Ireland Secretary Airey Neave was killed by a car bomb just weeks before a general election in which Margaret Thatcher came to power.

Before the Brexit referendum last June, British lawmaker Jo Cox was shot dead by a man motivated by Nazism. Both sides halted campaigning as a mark of respect. Pollsters, however, say the attack had little influence on the end result, which saw the country vote to leave the European Union. Then the campaign was suspended for a day.

Mr. Rahman said he expected the campaign to be suspended for longer than a day but certainly less than a week.