The shadow Brexit secretary, who is launching his leadership campaign in Manchester later, said the party need to “forge a path to victory” while being “radical and relevant”.
He also said he would personally take charge of the fight to stamp out anti-Semitism in the Labour Party if he wins the race to succeed Jeremy Corbyn.
Speaking ahead of the launch, he told Sky News: “We need to rebuild, and first and foremost we need to be united.
“We need to be an effective opposition, taking the argument to Boris Johnson – and that means having the best talent we have on show, but also being radical and relevant for 2024.
“But most importantly we need to forge a path to victory.
“We have now lost four general elections, and we now need to focus on the next general election, probably 2024, and set out our vision for the late 2020s and the 2030s – and a radical vision that will need to be.”
Sir Keir also said the party had not done enough to tackle the issue which bedevilled it under Mr Corbyn.
“We should have done more on anti-Semitism. If you are anti-Semitic you shouldn’t be in the Labour Party,” he told BBC1’s Breakfast programme.
“What I would do is lead from the top and say it’s my responsibility to deal with it. I wouldn’t say it’s for somebody else. I want the files, I want to know the numbers on my desk so that I can monitor this.
“Only when people who have left our party because of anti-Semitism feel that they can return will I be truly satisfied that we have dealt with the problem.”
Sir Keir has emerged as the clear favourite among Labour MPs and was the first of the six contenders to secure the 22 nominations required to progress to the next stage of the contest.
Shadow business secretary Rebecca Long-Bailey, the favourite of the left, and the backbenchers Lisa Nandy and Jess Phillips have also passed the threshold.
However shadow foreign secretary Emily Thornberry, with 10 nominations, and Clive Lewis, with just four, appear to be struggling.
They have until 2.30pm on Monday to get the necessary support or be forced to drop out.
Sir Keir would not be drawn into personal criticism of Mr Corbyn, although he said that it was right that he was now stepping down after Labour’s “devastating” general election defeat.
Unlike Ms Long-Bailey, who gave Mr Corbyn 10/10 and Ms Thornberry who gave him 0/10, Sir Keir refused to give him marks out 10 for his leadership.
“Jeremy Corbyn led us through really difficult times as a Labour Party. He positioned us in the right place on anti-austerity but we lost the election and now he is stepping down. That is the right thing to do,” he said.
“I am not going to get into ranking Jeremy Corbyn out of 10. I think it trivialises him. He is a friend and a colleague. I respect him, thank him for what he has done, but we are moving on now.”