By-elections are key tests of electoral opinion, a snapshot of the mood of voters which very often captures the frustration with a governing party.
Rarely however are they a pointer to what will happen in a general election scenario.
However, any vote in Lanarkshire in the coming months could test that rule of electoral politics.
The expected poll at Rutherglen and Hamilton West following the suspension of MP Margaret Ferrier from the Commons for 30 days, allows voters in the constituency the opportunity to demand a by-election.
Ferrier’s troubles of course stem from her egregious breaches of Covid rules. From the moment she lost the SNP whip and knew that she would be prosecuted she became a lame duck MP.
Her constituents, as evidenced in vox pop after vox pop have by and large lost confidence in her.