Washington (AFP) – President Donald Trump tore into his election challenger Joe Biden as a threat to the “American Dream” in a bruising speech Thursday accepting the Republican nomination for a second term against a backdrop of racial tensions and the deadly coronavirus pandemic.The former celebrity real estate developer spoke at a grandiose event staged at the White House — the first time a president has ever held a party convention at the executive mansion — and followed up with a vast fireworks display on the National Mall.In his 70-minute address, Trump went after Biden as hard as he cou…