Labour's deputy leader has warned of a "Margaret Thatcher-style landslide" for the Conservatives if his party fails to turn around current poll numbers.
Tom Watson said Labour had a "mountain to climb" to catch up with the Tories before 8 June's vote.
He told the Guardian: "It is going be very, very difficult... but we are determined to do it."
In her latest campaign visit, Theresa May accused Labour of "deserting" working class voters.
In his interview, Mr Watson asked for voters to consider that "a lot of local MPs are running on a good track record" when people head to the polling stations next month.
'Checks and balances'
Speaking on a tour of marginal seats in Wales, Mr Watson said Labour had "terrifically exciting" proposals in its manifesto - a draft of which was leaked earlier this week - but he was concerned about how far behind Labour were.
"If we get to 8 June and [Theresa May] still commands the lead in the polls that she had at the start of the election, she will command a Margaret Thatcher-style majority," said Mr Watson, referring to the Tories' 144- and 101-seat victories in 1983 and 1987 respectively.
Appealing for voters' backing, he added: "A Conservative government with a 100 majority... it will be very hard for them to be held to account in the House of Commons.
"It means there won't be the usual checks and balances of democracy... all those things go out of the window.
"You end up with governance by Theresa May without much accountability - and I don't think anybody wants that."
On Friday, Mrs May travelled to Tyne and Wear to appeal to an area that traditionally voted Labour.
"Proud and patriotic working-class people in towns and cities across Britain have not deserted the Labour Party - Jeremy Corbyn has deserted them," she said.
"We respect that parents and grandparents taught their children and grandchildren that Labour was a party that shared their values and stood up for their community.
"But across the country today, traditional Labour supporters are increasingly looking at what Jeremy Corbyn believes in and are appalled."
Mr Watson said the claim that the Tories are the party of the working classes was "the biggest myth perpetuated by London-based marketeers", and his party had to disprove it.