“The Tory manifesto has only just come out, and already it is being rewritten,” the paper wrote in an editorial. “Now ministers will struggle to explain where the cap will be set, how much it will cost — and why the manifesto originally said that previous proposals for caps on care costs were unfair.”

Critics tend to blame such mistakes on Mrs. May’s habit of trusting only a small circle of advisers, including her two closest aides, Fiona Hill and Nick Timothy. British news reports blamed Mr. Timothy for introducing the proposal into the manifesto at the last minute, without broader discussion among Mrs. May’s most senior colleagues.

But there may be an added element of political miscalculation at work. Given her earlier lead in the opinion polls, she appeared to have concluded that she could afford to seek a mandate for a measure that could be unpopular — including reducing some of the advantages enjoyed by older voters, whose incomes have been protected in recent years — while younger Britons have faced growing economic challenges, particularly with high housing costs and stagnant wages.

In addition to the changes to social care, Mrs. May proposed a less advantageous system for automatic raises in pensions, and plans to restrict help for older people with their heating costs to the poorest.

Such policies seemed devised to restrict welfare payments to those who rely on them and to redress intergenerational economic imbalances, and they are part of Mrs. May’s broader strategy, combining a strong role for the government in some economic and social areas with a hard-line stance against immigration and on leaving the European Union, or “Brexit.”

However, poll numbers show that the race is tightening, and Mrs. May’s manifesto not only gave the Labour Party some valuable ammunition but also prompted criticism from a wider pool, too. Charles Powell, who once advised Margaret Thatcher, described Mrs. May’s agenda as a “manifesto mugging of the elderly.”

The debacle also illustrated how the election campaign has so far been dominated by domestic issues, rather than serious discussion of how Britain should confront leaving the bloc, its biggest challenge in decades.

In the meantime, the European Union on Monday agreed on its strategy for exit talks, which are likely to begin within two weeks of Britain’s election on June 8.