What counts as a bad result for Boris in the by-elections?

The polls are open for the Wakefield and Tiverton and Honiton by-elections. The results are due in the early hours of Friday – with the Tories at risk of losing both of them. The votes have long been seen as a crunch point for Boris Johnson’s premiership – even though the fact that there was a confidence vote just weeks ago means that it is unlikely to lead to another.