Voters have begun heading to the polls after they opened at 7am this morning for the twin by-elections in Mid Bedfordshire and Tamworth.
Rishi Sunak is facing a major test, as he is being challenged on two large Conservative majorities.
MPs from across the spectrum have warned that it will be a disaster for the Prime Minister if he loses both seats - but even the loss of one seat would mean opposition parties have overturned a majority of at least 19,000.
Mid Bedfordshire, in which a by-election was called after Nadine Dorries resigned this Summer, has a Conservative majority of 24,500.
Mid Bedfordshire, in which a by-election was called after Nadine Dorries resigned this Summer, has a Conservative majority of 24,500
Meanwhile, Tamworth has a majority of 19,000. The by-election was called after Chris Pincher resigned amid a sleaze scandal.
Both sides have been attempting to manage expectations, with a number of Conservative MPs dismissing hopes of a by-election victory in Tamworth as a "lost cause".
One MP pessimistically told GB News that the party will "lose both" - predicting a catastrophic loss in Tamworth and Mid Bedfordshire.
However, some have expressed hopes of having a fighting chance in Mid Bedfordshire, with a former minister telling GB News there is "cautious optimism" in Dorries' former constituency.
But they dismissed chances of a victory in Tamworth, suggesting it would be a repeat of the 2019 Peterborough byelection which saw the Conservatives take third place behind Labour and the Brexit Party.
A Conservative Party insider told GB News the focus is currently on Mid Bedfordshire, saying that many have accepted that Tamworth is already gone.
They said that big losses at the by-elections will put Sunak in a "precarious position", explaining: "There will be a lot of worried MPs from across the party, who wouldn’t have expected to be worried a year ago."
But Labour has dismissed the remarks as "expectation management", with a party source saying that a double victory for the Conservatives is "really not out of the question at all".
They told GB News: "These are two safe Tory seats - 19,000 and 24.5,000 majorities. The idea that those majorities just fall away is fanciful.
"We're on the pitch. Which is a testament to how far we've come. We know it's really uphill though."
The source added: "The Lib Dems are campaigning very hard in Mid Beds. But they came third last time. They're third in the polls. And they're third with the bookies.
"So as far as we see it, a vote for the Lib Dems risks letting the Tories back in, in Mid Beds.