As the flag of a British workers union flapped behind him on a blustery June morning, Liam Kehoe was on strike with colleagues outside Royal Liverpool University Hospital, demanding better pay for porters, cleaners and catering staff. Their wages have failed to keep up with the surge in the cost of living, and many said they were living paycheck to paycheck.

Mr. Kehoe, 26, serves food in the hospital. On Thursday, the day of Britain’s general election, he plans to vote for the center-left Labour Party because of the economic situation and the crumbling state of the National Health Service, he said.

Thinking of the life that his parents built on salaries earned as a nurse and a truck driver, Mr. Kehoe says that young people have been left with far worse prospects after 14 years of a Conservative-led government. “If you go back 30 years ago, houses were a bit more affordable, life was a little bit easier,” he said. “Nowadays, it’s like you can’t afford anything.”

Polls suggest more than half of voters under 35 plan to vote for Labour on Thursday, compared with 27 percent of voters over 65. While the gap between young and old in politics is not new, the extent of the split in Britain in recent years is exceptional, with support for the governing Conservative Party dropping sharply in all but the oldest age group, according to recent polls.

Before 2019, the major factor in whether people voted Conservative or Labour was income. More recently, “age has replaced class as the defining way in which people vote,” said Molly Broome, an economist with the Resolution Foundation, a British research institute.