Shadow business secretary Chuka Umunna has accused his Labour Party colleagues of "behaving like a petulant child who has been told you can't have the sweeties in the sweet shop".
The Streatham MP said Labour should not be "screaming at the electorate" after its poor general election performance.
He spoke out after Labour was hit by a row over its stance on welfare cuts.
He also criticised Jeremy Corbyn, amid reports that he is doing well in the race to be Labour leader.
In an interview with BBC Newsnight political editor Allegra Stratton, Mr Umunna said Labour were "sent a very strong message" by the electorate at the general election, where it was almost wiped out by the SNP in Scotland and failed to make ground on the Conservatives in England.
"We're running around stamping our feet, screaming at the electorate when ultimately what we need to do is meet people where they are at, not necessarily where we would want them to be," he said.
Labour has been divided by how to respond to the welfare cuts set out in George Osborne's Budget, with some MPs reacting angrily when acting leader Harriet Harman said it should not oppose all of the measures.
The party's new leader will be announced at a special conference in September.
Mr Umunna, who was briefly in the leadership contest before withdrawing, said the Conservatives would want Mr Corbyn to win.
The odds on Mr Corbyn winning the leadership have recently been reduced, though he only made it on to the ballot paper after some MPs not thought to be his supporters agreed to make up the necessary number of nominations for him.
Mr Umunna said he had nothing against the Islington North MP but added that his political views were not "a politics that can win".
He said Mr Corbyn was "weak on defence at a time when global insecurity is rising" and backed "more generous social security payments for people who can work but refuse to work".
"I'll tell you what, there is no glory in opposition," Mr Umunna added. "Ultimately we will betray our people if we don't get elected."