Lisa Nandy called on Labour members to make "the brave, not the easy choice" in the coming leadership contest.
Speaking in Dagenham, east London the Wigan MP said the party was "in retreat".
On Brexit she said that a Labour leadership divided between Leave and Remain had left candidates in an "impossible position".
She added: "The choice: to be for Labour, or to be for your community.
"You could not be both."
"I promise you that under my leadership this will never, ever happen again."
Her speech, which came as MPs and MEPs nominations closed, saw Ms Nandy reject calls that the party should be forced to choose between voters in big cities and those in Labour's traditional heartlands.
She told activists: "The stark reality is the path back to power runs not along our red wall but across a red bridge that connects our towns and cities and stretches from Dagenham to Fulham, Aberdeen to Glasgow, and Cardiff to Wrexham."
She slammed those who had accepted that Labour must choose between progress and tradition, describing it as a lie "peddled by those who seek to defeat us, that we are locked into a tug of war and we must choose: between working and middle class, leave and remain, north and south, young and old, towns and cities."
She added: "But I do not accept this. For all of the radicalism and energy of our recent years, they are wrong to believe that this is a zero-sum game."
Ms Nandy quoted Labour legend Barbara Castle as she told supporters “in politics, guts is all".
