The former shadow work and pensions secretary warned that Labour was "at a crossroads" between reuniting the party or "satisfying ourselves with ongoing division and opposition".
"It's the clearest choice we have had to face in a generation. A choice that will not just determine the future of our party, but the future of the millions of people in Britain who need Labour in power," he added.
Smith, who was little known outside of Westminster before the election, said he was "proud" to bring forward a "positive programme" of policies, including a pledge to introduce a £200m investment programme.
But Smith is expected to lose the election to Corbyn, who attracted almost 60% of the vote in 2015.
The bookmakers have the Labour leader as favourite, and he has attracted the most Constituency Labour Party nominations and endorsements from affiliated trade unions.
The latest opinion poll from YouGov for The Times, of more than 1,200 people between 25 and 29 August, put Corbyn 24 points ahead of Smith (62% versus 38%).
A defiant Smith, however, told IBTimes UK on Friday: "Don't write me off until the final whistle, is all that I ask." The "final whistle" will be around midday on Saturday when the winner of the leadership contest is announced at Labour's annual conference in Liverpool.
This article was first published
on September 20, 2016