Val Demings, a Democratic representative from Florida among contenders to be Joe Biden’s presidential running mate, has castigated Donald Trump for having the “gall and nerve” to use a gaffe by Biden as a weapon on the campaign trail. Biden apologised on Friday, for saying that if African Americans “have a problem figuring out whether you’re for me or Trump, then you ain’t black”. The remark prompted gleeful tweets from Trump, fierce attacks from supporters of the president and criticism from Biden’s own backers. “The vice-president shouldn’t have said it,” Demings told CNN’s State of the Union on Sunday. “But...