Sir Keir Starmer has called on Labour members to end factionalism and promote a unity, as he formally launched his leadership campaign at the Mechanics Institute in Manchester on Saturday afternoon.

Speaking at the birthplace of the Trades Union Congress (TUC) Starmer said: “We need to fight to regain our heartland seats, not just those lost in this election but the ones lost in previous elections. But we have got do more than that because if we just re-win our heartland seats we will lose the next election.

“We have to win seats in Scotland where we went down to one MP. We need a strategy for Scotland so we can win again. In 2017, we had seven MPs and we had 20 seats within 2,500 votes of winning and then we went backwards.

“We need to address how we win more seats in Wales. I was in Wales last night where the hurting of the Labour party is very real in relation to seats that we should have won.

“And if you draw a line from London to Bristol and look south, there are over 120 seats and we the Labour party have a handful. So we need to win in the south, the south-east and the south-west.

“We’re focusing on the last general election, (but) we have now lost four general elections in a row. The next one will probably be in four or five years’ time and if we lose that we will have the longest period of the Labour party out of power since the second world war. We have a mountain to climb.”

To applause from a packed room of party members and activists, Starmer said the first thing needed in Labour’s fight for economic, social and climate justice was to be “united as a party”. “We cannot fight the Tories if we are fighting each other,” he said. “Factionalism has to got to go.”

He said Labour also needed to be a “very effective opposition” against prime minister Boris Johnson who he said was “a man of no principles, no moral compass, who will go anywhere to stay in power”.

But a Labour party in opposition is not changing lives, he went on, and the “huge task” ahead was to forge a way forward to victory and success.

Starmer said: “Jeremy Corbyn made our party the party of anti-austerity and he was right to do so. He made us the party that wanted to invest more heavily in our public services and he was right to do so. We must retain that. We build on that and don’t trash it as we move forward.

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Earlier, the shadow Brexit secretary had vowed to lead the struggle against antisemitism in Labour. Speaking on BBC One’s Breakfast programme, Starmer said: “We should have done more on antisemitism. If you are antisemitic you shouldn’t be in the Labour party.

Starmer said he had argued within the party for tougher action. “I argued for automatic expulsion. It seemed to me that if you have been chucked out of the Labour party for supporting another political party, you should be chucked out for being antisemitic,” he said.

According to the latest Labour figures, only Clive Lewis and Emily Thornberry do not have the required 22 nominations from Labour MPs to reach the next stage of the process to replace Corbyn. Thornberry has 10 and Lewis has four.

Starmer has 68, followed by Rebecca Long Bailey with 26, Lisa Nandy with 24 and Jess Phillips on 22.