Ed Miliband will outline his plans to end exploitative zero-hours contracts during a visit north of the Border today to appeal to Labour voters flirting with independence.
As the Labour leader's Shadow Cabinet meets at the Emirates Arena Commonwealth Games venue in Glasgow, Mr Miliband will attempt to woo voters with the message that his party outflanks the SNP on the Left.
Mr Miliband will say his plans to tackle controversial contracts show that social justice, a key theme of the SNP's push for independence, can be better achieved by remaining part of the UK.
He will appeal to Scots that while a Yes vote in September's referendum would be for life, the looming 2015 General Election means the Conservative-Liberal Democrat Coalition could be "just for one more Christmas".
He will also suggest that the nationalists do not have a monopoly on looking after Scotland, saying he cares deeply about the outcome of the poll.
The announcement marks a stepping up of the campaign to win over undecided voters, many of whom are thought to be former or current Labour backers.
A party source said: "We are taking the fight to where we know there are Labour voters who may be undecided and who we know we need to convince in September."
Ahead of his visit, the SNP challenged Mr Miliband to pledge that a future Labour government would not cut the amount of money Scotland receives from the Treasury by scrapping the Barnett Formula.
Yes campaigner and former Labour MP Dennis Canavan has also called on former Labour Prime Minister Gordon Brown to agree to a public debate on independence and "how we can best achieve the fairer and more socially just society we both seek".
Mr Miliband's choice of the 2014 Games' main sporting venue for the meeting is likely to be viewed as a riposte to Prime Minister David Cameron, who chose the Olympic Park in London for a recent speech on independence.
The Opposition leader will go on to hold a public meeting in Motherwell with Scottish Labour Leader Johann Lamont and Shadow Scottish Secretary Margaret Curran.
He is expected to outline his plans, to be contained in Labour's election manifesto, to give employees the right to demand fixed-hours contracts after six months.
Labour would also outlaw the zero-hours contracts that prevent staff working for other employers and recommend a right to compensation when shifts are cancelled at short notice.
Up to one million employees are thought to be on zero-hours contracts across the UK, including 90,000 staff in Scotland.
Unions warn the contracts exploit staff by offering no guarantees of work or wages. But business leaders defend them, saying they appeal to many workers and help create jobs.
The SNP have said that independence could help end the abuses of zero-hours contracts.
Mr Miliband said: "I don't believe you can serve the cause of social justice and that of narrow nationalism.
"If Scotland was to separate from the rest of the UK, our country would be in a race to the bottom with the nationalists competing on lower tax rates for the top and lower wages or living standards for everyone else.
"That's why working people in the UK will be worse off without Scotland and working people in Scotland will be worse off without the UK."
He added: "I want to rebuild our country in the cause of social justice, make the changes needed to tackle the cost-of-living crisis, and create a more successful economy that works for hardworking people in Scotland and across the UK.
"I don't have a vote in September, but I care deeply about the decision people across Scotland will make."
Mr Miliband is also expected to say today that narrow nationalism would force Scotland "into the same old Tory economic strategy that has failed Scottish families and families across the UK", adding that his message to Scots is that "by working together we can ensure that the Tory government in Westminster is just for one more Christmas. But independence would be forever".
Labour said the shadow cabinet would be taking the party's positive message over 500 miles across Scotland from the Highlands to the Borders today. Shadow home secretary Yvette Cooper will visit doorsteps in Greenock, Inverclyde.