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