Warsi’s move from Leave to Remain came as no surprise to pro-Brexit side

In normal circumstances, the decision by a Conservative peer with a patchy political record to switch sides in Britain's EU referendum campaign would have a limited impact on the campaign. Sayeeda Warsi's announcement that she had transferred her allegiance from Leave to Remain came as a surprise to the pro-Brexit side, not least because she had never actually campaigned with them.

Warsi had made a number of public statements backing Brexit but she declined to play an active role in the campaign or to appear at Leave events. But her denunciation of the “nudge-nudge xenophobia” of the Leave campaign was a direct hit at the heart of its greatest vulnerability as the referendum approaches.

Vote Leave was determined from the outset that Nigel Farage should not play a leading role in the campaign, seeking to sideline the Ukip leader in favour of more mainstream figures such as Conservatives Michael Gove and Boris Johnson and Labour's Gisela Stuart. When ITV chose Farage to go up against David Cameron in consecutive televised forums, Vote Leave accused the broadcaster of joining the Remain campaign.

As the campaign progressed, however, and the issue of immigration became the rocket fuel propelling Leave's poll numbers, it was more difficult to keep Farage out of the picture. After all, the spurious arguments now advanced by Vote Leave, about the imminent threat of mass migration from Turkey and the burden EU migrants placed on British public services, were those Ukip had been ventilating for years.

Farage says his notorious "Breaking point" poster, showing a queue of Syrian refugees at the Croatian border, was only designed to be shown for one day. Unfortunately for the Leave campaign, that day was last Thursday, the day Labour MP Jo Cox was murdered outside her constituency office.

Farage on Monday accused Cameron of seeking to politicise the murder, but the truth is that the Remain campaign doesn't have to, because the event itself has changed the atmosphere ahead of the referendum.

Last week, when Leave was powering ahead in the polls, Farage said it was because “I think collectively people are beginning to put two fingers up to the political class”. Maybe they were. But as they mourn the loss of a politician who was so evidently motivated by serving others, voters are now less likely to approach Thursday’s vote in a spirit of anger. And that is bad news for Leave.