The SNP's Nicola Sturgeon asked whether there was anything Ukip leader Nigel Farage would not blame on immigration.
Farage pulled a face in response - which appeared to suggest he did not disagree with the statement, ITV News political commenters said.
Sturgeon "there's nothing Nigel Farage wouldn't blame on foreigners" Farage looks as if he would have to think about it. #leadersdebate
Farage seems pretty happy with Sturgeon's suggestions that there is nothing he won't blame on foreigners. #leadersdebate
Last night there wasn't one particular knockout blow but the political landscape in Britain does seem to be shifting.
Another day on the election trail and the campaign catchphrase is starting to bite.
Leaders' Debate shows was how our political horizon has suddenly changed as smaller parties did all they could to make themselves heard.