Labour leadership candidate Rebecca Long-Bailey will promise to “back workers in every dispute and strike against unfair, exploitative and unjust employers”.
Speaking at a rally in Sheffield she will warn her party not to treat trade unions like “embarrassing relatives”.
Lisa Nandy, Sir Keir Starmer and Emily Thornberry also remain in the race to replace Jeremy Corbyn.
The results of the contest are due to be announced on 4 April.
With two weeks to go until voting opens, leadership hopefuls have been setting out their pitches to Labour members.
On Thursday Ms Nandy called for billions of pounds to be given to local councils rather than LEPs (local economic partnerships).
And Sir Keir said promoting “national wellbeing” should be as high a priority as delivering economic growth.
Ms Thornberry proposed seizing empty homes to ease homelessness and housing problems.
Mrs Long-Bailey will use her speech to argue that “standing on the side of workers and trade unions, no questions asked, is going to be crucial in standing up to this reactionary Conservative government.”
She is expected to tell supporters that the party needs a leader who is “as comfortable on the picket line as at the dispatch box” and will vow that “under my leadership Labour will never return to condemning striking teachers or firefighters.”
Her comments may be seen as a pointed criticism of Ed Miliband, the last Labour leader but one, who came under attack in 2011 for failing to support strikes by public sector workers.
The shadow business secretary will argue “the Labour Party is the parliamentary wing of the whole labour and trade union movement, and our path to power is in rebuilding it.”
She will also promise to commission a trade union recruitment plan “targeted in red wall seats in which we need to rebuild”.
And if elected to government she would seek to ensure the trade union movement grows by over a million.
The 2019 general election saw Labour lose a number of their long-held seats across the Midlands and north of England – known as the “red wall” to the Conservatives.
Speaking earlier on the Victoria Derbyshire programme, the Salford and Eccles MP also said she wanted to give workers the right not to be contacted on work-related issues outside of normal working hours.