Donald Trump has dismissed Barack Obama’s time in the White House as a “disaster” after the US president said he was not fit to succeed him.
“He’s been weak, he’s been ineffective,” Republican candidate Mr Trump said of Mr Obama in a Fox News interview on Tuesday.
Mr Obama has questioned why Mr Trump’s party hasn’t disowned him.
Mr Trump has also turned on two senior figures in his own party who have publicly criticised him.
In an interview for the Washington Post, he refused to endorse House Speaker Paul Ryan and Senator John McCain, who are up for re-election in November.
Republican donor backs Clinton
Amid the feuding within Republican ranks, prominent party donor and fundraiser Meg Whitman has publicly endorsed Democrat Hillary Clinton, saying Donald Trump’s “demagoguery” had undermined the national fabric.
“To vote Republican out of party loyalty alone would be to endorse a candidacy that I believe has exploited anger, grievance, xenophobia and racial division,” she wrote on Facebook.
“Trump’s unsteady hand would endanger our prosperity and national security. His authoritarian character could threaten much more.”