Mike Amesbury won the seat of Runcorn & Helsby at the 2024 general election with a 14,696 majority – one of the top 50 largest parliamentary majorities achieved by Labour on polling day.
He took the constituency with 53% of the vote, with Reform in a distant second place on 18%, followed by the Conservatives (16%), the Greens (6%) and the Liberal Democrats (5%).
Runcorn & Helsby is a new seat that was fought for the first time at the 2024 election.
Had the constituency existed at the previous general election in 2019, notional data suggests Labour would have won but on a slightly lower share of the vote (49%) than Mr Amesbury achieved in 2024, with the Tories in second place.
Based on the 2024 result, Reform would need a swing in the share of the vote of 17.4 percentage points to gain Runcorn & Helsby.
This is the equivalent of a net change of 18 in every 100 people who voted Labour in 2024 switching to Reform.
The largest swing at a by-election in the 2019-24 parliament where a seat changed hands was 34.1 percentage points, from the Conservatives to the Lib Dems in North Shropshire.