Labour needs a Yes vote in next month's assembly powers referendum if it is to win back an assembly majority, says Welsh Labour leader Carwyn Jones.
At a rally on the eve of Welsh Labour's conference in Llandudno, he urged party members to campaign for more powers for the assembly.
The 3 March vote was one of the most important in Welsh history, he said.
Labour leader Ed Miliband told party members the referendum was "a logical step" for Welsh devolution.
Voters will be asked to decide whether the Welsh assembly should have more law-making power.
Mr Jones said: "We are less than a fortnight away from one of the most important votes Wales has faced in its history.
"I don't believe we will get a majority [in the Welsh assembly] unless we win and get a Yes vote on 3 March.
"We need to make sure, as I said, that when we win on 5 May [the Welsh assembly elections] it's a stepping stone towards winning across the whole of the UK in 2015."
He said the assembly's current system for passing legislation had worked "reasonably well, certainly at the beginning".
As things stand, before new legislation can be put forward in the assembly, Parliament must give its authority to devolve powers in that area.
Mr Jones said that over time, delays had crept into the system.
He said it had taken three years to draw down powers over affordable housing and four years for powers over school transport.
The UK government had raised a series of "barriers" to the assembly government's recent bid for powers over organ donation, he said.
He added that the assembly needed more law-making powers to "shield ourselves from the worst excesses" of the UK government.
He said No campaigners had presented "false arguments" about the referendum.
He said there was no need for more AMs, and that the assembly could "manage with 60".
"We have got 13 days to begin the process that ends with Ed Miliband becoming prime minister of the UK," he said.
Mr Miliband appeared at the rally alongside Mr Jones and said the first minister was "giving the lie to the Tory idea that there is no alternative".
"I am here to support with all my heart a Yes vote in the referendum on 3 March," he said.
Mr Miliband said that doubters about devolution at the time the assembly was established had been proved wrong.
He said: "Who now says let's go back to the old system of running Wales from London?"
Wales had "led the way" under devolution by introducing a children's commissioner and concessionary bus fares - policies that were later taken up in England, he said.
Mr Miliband said the Labour Party "were not separatists, we believe in Wales as part of a strong UK".
He said that the referendum was "a logical step and it's a logical step because it's based on trust in the people of Wales and trust in the Welsh Assembly Government".
Opponents of the Yes campaign say giving more powers to the assembly means more money for politicians in Cardiff Bay whose track record is one of failure.
Rachel Banner, of the True Wales group, which opposes further devolution, has said: "It is not unusual for politicians to be demanding more powers for themselves.
"We say that the issue of direct law-making power will do nothing to address the disastrous problems that we have in Wales after 12 years of devolution."