Donald Trump will formally accept the Republican presidential nomination on Thursday, at a time when his Democratic rival Joe Biden is struggling to fight off more doubts about his ability to run a successful campaign and serve another four-year term. Trump will deliver a speech to a packed arena of Republicans in Milwaukee before the party concludes its four-day national convention. When accepting his third consecutive presidential nomination from the party, just days after surviving an assassination attempt during a rally, Trump is expected to call for national unity to make the United State…