Leave campaigners are the "grave-diggers of our prosperity" and will have to answer for their lies during the referendum campaign, Sir John Major has said.
In a clear swipe at Tories Boris Johnson and Michael Gove, the former prime minister said David Cameron had faced abuse from people "we might have expected better from".
Quitting the European Union will not stem immigration and risks creating a broken Britain, he insisted.
Leaving would be a "disproportionate" response to migration concerns and the country would live to regret it for a "long time to come".
During a campaign visit to Bristol with Mr Cameron and Labour's Harriet Harman, Sir John warned Britain not to "pull up the drawbridge".
He said: "If our nation does vote to leave, we must respect their decision, but if they vote to leave on the basis of half-truths and untruths and misunderstandings, then pretty soon the grave-diggers of our prosperity will have some very serious questions to answer."
"They will have to account for what they have said and done but that will be of little consolation for we will be out, out for good, diminished as an influence on the world. A truly Great Britain shrunk down to a little England perhaps without Scotland, perhaps with a grumpy Wales, and certainly with a Northern Ireland divided from the south by the border the controls that would then be the edge of the European Union."
Dismissing accusations that the Remain campaign had been "project fear", he insisted that Mr Cameron had a "duty" to to warn the nation of the dangers of quitting.
He said: "In return for doing that he has faced a great deal of opposition and sometimes abuse from people who, frankly, we might have expected better from.
"I think the way in which he has conducted himself in putting the country before self and the country before party is quite remarkable."
Sir John went on: "In a world that is on the move in almost every part of the world, leaving the European Union is no solution to the scale of migration around the world.
"It is a reaction to it, but it is not a solution to it and to try and solve a short-term problem that will diminish as the European economy begins to grow again and leave Europe and risk a far greater long-term impact to our prosperity in the world is quite disproportionate to the problem that we face."
He attacked the "absurdity" of suggestions that huge numbers of Turks would head to Britain if Turkey gains EU membership and claimed they would earn a better wage in France and Germany.
He added: "We would also be diminished as a country. I don't want to see a broken Britain with less influence, less importance in the world and yet that is what we risk if we leave."
Mr Cameron told the crowd in Bristol that the Remain campaign could be summed up in the single word "Together".
"If we want a bigger economy and more jobs, we are better if we do it together," said the Prime Minister.
"If we want to fight climate change, we are better if we do it together. If we want to win against the terrorists and keep our country safe, we are better if we do it together.
"So please give it everything you've got in these last hours to make sure that people go out and vote tomorrow - go out and vote Remain for a bigger, better Britain inside a reformed European Union. Stronger, safer, better off. A day to go. Let's do it."
The PM said the range of support that Remain had garnered across the political spectrum was a telling point in its favour.
"We've got the Green Party, we've got the Liberal Democrat party, we've got the Labour Party, we've got a Conservative Government and Prime Minister, we've got the trade unions, we've got business large and small all saying we are stronger, we are safer, we are better off if we stay," he said.
Standing alongside Mr Cameron and Sir John, Ms Harman said: "It's an irrevocable decision and I hope that people, when they vote tomorrow, will have in their hearts the knowledge and the belief that we are better as a country when we are outward-facing, when we are inclusive, when we are confident and looking to the future. So let's vote Remain tomorrow."
Setting out Labour's case for Remain, Ms Harman said: "For people in this country, the EU means jobs, it means investment, it means the opportunity to export what we manufacture and the services we provide.
"We want, as Labour people, Britain to stay in the EU because we want more jobs for the future."
And she added: "The EU has given people more rights in the workplace and we want to strengthen those rights."
Mr Cameron's mini-tour is taking in a variety of venues including a housing development, farm, school and nature reserve as he seeks to present a case that Brexit would harm every part of society.
Individuals including builders, doctors, nurses, farmers, veterans and business people - as well as prominent politicians from other parties - are among those joining the bus at points along the way to travel to a closing rally.
After touring a housing development near Swindon, the PM welcomed construction apprentice Maisey Capen on board.
Paediatrician Xand Van Tulleken is also among those travelling on the bus.
"It's obviously a very close fight and I am putting everything into it," Mr Cameron said.
"Today we are going round and getting people - nurses, doctors, farmers, small businesses, entrepreneurs - coming into this bus, explaining why they want to stay in.
"Because there is an amazingly wide range of people."
Mr Cameron joined forces with ex-Liberal Democrat leader Paddy Ashdown to chat with military veterans at a farm shop in Oxfordshire.
Among them was 92-year-old Patrick Churchill who served with the Royal Marines at the D-Day landings.
He told the PM that he saw immigration as a "great problem" but that Brexit - with the perceived economic risks - was not the answer.
"A great problem is the immigration and all that," he said. "But you cannot solve that overnight and you have to make sacrifices one way or another.
"My worry is what is going to happen to these young people when they are married, they have sons and daughters and grandsons and granddaughters. What will happen in 10 or 20 years? Where are the safeguards?"
At Warriner School in Bloxham, Oxfordshire, the PM high-fived cheering pupils before talking to sixth formers voting for the first time tomorrow.
Only one of the group admitted to being set to back Brexit - a frankness called "brave" by ex-special forces soldier Lord Ashdown who had told them he was "trained in cruel and unusual punishment".
After the school, the growing party was due to visit a hospital and a large business before heading for the rally, which will feature speeches from figures from across the political spectrum and some of the individual Remain backers who Mr Cameron met up with on the way.