A union boss has said Sir Keir Starmer must "cosy up to working class people" to be effective in Opposition against the Conservative government.
Speaking to Sky News' chief political correspondent Jon Craig ahead of Labour's party conference, RMT chief Mick Lynch said Sir Keir must "show a way that he identifies with the struggles working people have got".
It comes as rail workers prepare to continue strikes next week in their long-running dispute over pay, jobs and conditions.
Mr Lynch said he does not expect Sir Keir to join a picket line himself, but must oppose the "anti-trade union laws" that the Conservatives are planning to introduce.
"We certainly want to see a bit more robust defence of working people in this time of hardship," he told Sky News.
"Labour have to show that it's on the side of ordinary people in this country who are realty struggling.
"Keir Starmer has got to show a way, don't expect him to come onto our picket lines, I'm not that naive, but to show a way that he identifies with the struggles working people have got."
Mr Lynch continued: "These new anti-trade union laws are a real pinch point for him, he's got to stand up in the commons and at these conferences and say I'm going to oppose these laws.
"We're going to be faced with imposed poverty, and there's a danger there'll be no opposition, either a political opposition, an opposition in the workplace through the unions, or on the streets through the right to protest.
"We want him to win the election but we want him to do it on a basis working people can get behind - he can't cosy up to business and the Daily Mail all the time, he's got to cosy up to working class people."
Meanwhile, speaking at a The World Transformed fringe event alongside Mr Lynch, Labour MP Zarah Sultana said she will not listen to those who say MPs should not join picket lines.
"Not everyone in the Labour Party is clear about the class they represent so it's good to remind them about that... we're the Labour Party, the clue is in the name," the MP for Coventry South said.
"No Blairite can tell me to not go on a picket line because I'm not going to f****** listen."
The next strikes by rail workers are due to take place on 1 and 5 October - at the beginning and end of the Conservative's party conference in Brighton.
Probed on the timing of the strikes, Mr Lynch replied: "Well we've got to make the action as effective as possible. There's never a good time to have a strike... whatever we do we'll be criticised.
"We've got to pursue our suit as best we can. What I'd like to do is get a settlement and suspend that action."
Arriving at Labour's party conference in Liverpool earlier, Sir Keir vowed to set out a plan for "an economy that works for working people" - unlocking "growth for everyone".
But the event comes as the Labour leader faces questions about his position on shadow ministers joining picket lines.
It followed the sacking of shadow transport minister Sam Tarry after he gave a series of interviews from one.
Labour was born from the trade union movement and still benefits considerably financially from its affiliation to the unions - although not from RMT.
The Labour Party says this year's conference will welcome more than 12,000 attendees for four days of policy discussion, training and events.