Harkins' 58th minute strike cancelled out Sammy Clingan's early opener for the Spireites and the draw is the least Blackpool deserved after an impressive second-half display.
It looked like Chesterfield, who started the game in fourth place, were set to continue their recent good form when Clingan fired a low free-kick past Lee Jones from the edge of the box in only the ninth minute.
John Murphy then saw an effort kept out by the legs of Barry Roche before both sides were then reduced to 10 men four minutes later when Chesterfield skipper Shane Nicholson and visitors' midfielder Ciaran Donnelly were sent off after a scuffle.
Nicholson fouled the Blackpool man then appeared to head butt him when Donnelly angrily retaliated.
The disruption seemed to suit Simon Grayson's men better as the Saltergate side lost their shape.
And Blackpool were level 13 minutes into the second half. Harkins volleyed home from 10 yards after Reuben Hazell's goalline clearance from Lewis Gobern flew straight to him.
Caleb Folan and Alex Bailey came close to winning it for Chesterfield in the last five minutes but defeat would have been harsh on Grayson's battling side.