Prof Tonge said he sees the second vote as crucial because in the most recent council elections across Greater Manchester “no party came close to winning 50% of the vote; Reform led on 31%, followed by Labour on 24% and the Greens on 19%”.
He said it was not possible to predict how votes could be transferred as “none of the three previous mayoral contests in Greater Manchester has ever gone to round two.”
Tonge said Reform would have been favourites if the voting system remained first-past-the-post, but not with SV.
“Some transfers to Reform’s candidate, Sian Astley, may come from the Conservatives, distant runners-up last time but trailing in the one opinion poll to date.
“Bad blood between the leaders of Reform and Restore Britain, Nigel Farage and Rupert Lowe, evident in the recent Makerfield by-election, may prevent many Restore voters offering second preferences to Reform.”
He said in contrast Labour’s Bev Craig is a “potential recipient of a wider range of transfers, from Greens and Liberal Democrats most probably, which may get her over the line”.
Tonge added: “The Greens have tried to claim this is a contest between them and Reform – which given Labour’s historical strength across Greater Manchester and possible Burnham fillip – is clearly not the case.
“However, the party’s candidate, Geraldine Coggins, may do very well with transfers from other party supporters – if – big if – she can survive into the second round.”
All the candidates standing in the by-election will be announced at 16:00 BST on Monday.