Labor leader Corbyn insists he backs Britain's nuclear deterrent
May is still favorite to win in the June election with an increased majority.
But Corbyn hopes her manifesto plan to end a guaranteed so-called triple lock annual rise in pensions, will see more elderly people switching to Labor.
Speaking at the International Convention Center in Birmingham he described May's manifesto as the most divisive for years, pitching young against old.
"Their manifesto is a typical nasty party attempt to set generations against each other. For pensioners they offer a triple whammy of misery," said Corbyn.
May, meanwhile, spent the day electioneering in West London where there was strong support for Britain staying in the EU.
She said it was time for people to put divisions behind them, telling voters Britain needed strong and stable leadership. She repeated her message Labor could not be trusted to run the country.
The latest opinion poll carried out by Orb for the Daily Telegraph Saturday put the Conservatives 12 points ahead of Labor with Corbyn's party gaining some ground in the past week.
The poll, carried out a few days ago, put the Conservatives on 46 percent, unchanged from a week ago, with Labor gaining two points to reach 34 percent.
