"I will not back down,” U.K. Prime Minister Boris Johnson said, as he delivered his latest threat to walk away from Brexit talks without a new trade deal.

For economists, business leaders and European Union officials, the question is whether to believe him. There are plenty of reasons why they should, despite the political dangers for Johnson of worsening the already severe economic damage the U.K. faces from the coronavirus pandemic.

First, for Johnson, Brexit is personal. He led the campaign to exit the EU in 2016, then won a big majority to deliver Brexit in last December’s election. Completing the process and taking Britain out of the EU’s legal orbit at the end of the transition period in December would be his legacy.