So, what are they saying to potential voters?
After two bruising years in government, Labour is desperate to hang on to Gorton and Denton.
That ambition perhaps will not be helped by the situation in which Gwynne stepped down.
Gwynne, who was a health minister, had been on sick leave for months after being suspended over his involvement with the Trigger Me Timbers Whatsapp group, in which he and local councillors made offensive comments about residents and colleagues.
Nationally, the party’s been on the ropes with the fallout from the Peter Mandelson scandal and questions about Sir Keir Starmer’s future as prime minister.
Add into the mix the controversy over Greater Manchester Mayor Andy Burnham being blocked from standing as Labour’s candidate, and you can see why the city councillor they chose instead, Angeliki Stogia, is saying she is focused on local issues, unity and fairness.
Labour insists this by-election is a “two-horse race” between them and Reform UK, and accuses Nigel Farage’s party of being divisive and “far right”.
Its candidate, academic-turned GB News presenter Matt Goodwin, suggests he is on the brink of one of the “biggest shocks in British political history”.
He argues Labour have taken the area for granted for decades and says many residents agree with his anger over the benefits bill and illegal migration.
Reform UK is pitching this by-election as a referendum on Starmer, telling those who are unhappy with the prime minister that this is their chance to make that point and potentially force him out.
Farage has been frequently and enthusiastically visible so far during the campaign.