Here’s some useful data on that from Will Jennings (full piece, which is well worth a read, here):
As he writes:
Historically, by-elections have offered a yardstick for the future electoral prospects of governments. Every government tends to lose support in by-elections, but those that repeatedly lose badly tend to fare worse at subsequent general elections.
He also adds:
A defeat in Tiverton & Honiton would be catastrophic. As well as signalling the depth of disillusionment with the PM and his party, it would further establish the trend of the Lib Dems inflicting damaging losses on the Conservative Party.
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