A week ago, in the Richmond Park by-election, Remainers took their revenge. The Liberal Democrats overturned Zac Goldsmith's elephantine 23,015 majority by turning the contest into a referendum on Brexit (the constituency voted for Remain by 72-28). Today, in the Sleaford and North Hykeham by-election, Ukip aim to do the same - but from the reverse position. The seat, where the party finished third in 2015, was 61.5 per cent for Leave.

There is no prospect of a Ukip victory. The Conservatives currently hold a majority of 24,115 and Theresa May's "hard Brexit" stance (which prompted the resignation of the seat's MP Stephen Phillips) has attracted anti-EU voters. But Ukip, which was just 974 votes behind Labour in 2015, will likely finish second. New leader Paul Nuttall's ambition to "replace" the opposition demands no less. Just as the Tories' support for a hard Brexit insulates them from a Ukip challenge, so Labour's support for a softer version (including free movement) makes it vulnerable. The Liverpudlian Nuttall aims to win seats off the party by exploiting the divide between the party and its working class voters. Labour MPs deride Ukip's populist pretensions (noting that Nuttall once supported NHS privatisation). But they once similarly mocked the SNP as "tartan Tories".

Mindful of this, Labour MPs are taking the threat seriously. Even those with majorities traditionally weighed, rather than counted, worry Ukip could sweep them away ("there's no safe seat outside of London," one said). As I write in my column this week, Labour MPs fear Brexit could realign British politics along Remain-Leave lines. The Lib Dems will be the champions of the former, with Ukip the champions of the latter. The Tories, a Labour MP says, will stand above the fray with "the only viable prime minister". Meanwhile, the SNP will remain hegemonic in pro-Remain Scotland. "We face a tougher electoral map than at any time in our history," Jonathan Reynolds, the shadow Treasury minister, told me. Many expect Labour to finish fourth in Sleaford as Remainers defect to the Lib Dems.

To some, however, the potential for Ukip gains appears limited. The party finished second to Labour in just 44 seats in 2015. It was less than 10 points behind in only one of these and less than 20 points behind in just 14 others. But having seen their Scottish colleagues eviscerated, Labour MPs are loath to describe any swing as "impossible". Ukip could indirectly cost the party seats by attracting defectors in Tory-Labour marginals (witness Ed Balls's fate in 2015). Labour's poll ratings averaged just 29.5 per cent last month. But MPs fear this is merely "the tip of the iceberg". At this point in previous parliaments, the party's support has only ever fallen.

In response, Labour MPs are taking drastic action. "People will follow the Lib Dem playbook, treat the party as a franchise and run ultra-local campaign," says one. Leaflets will be free of references to Corbyn and national policy. “You’ve got to cut the mother ship adrift and row yourself to safety. It's every man for himself now."